17 years on and still no solution to AV Amp delays!

I’m going to hazard a guess that it is because it undermines the entire reason the Sonos system exists? Essentially, that delay allows the entire “whole home system” that Sonos has based their business on to play in sync across all speakers. Rewriting the base software/firmware of their entire system might be challenging, in order to deal with a minority of users. 

Is it not ideal? Sure. But you have to consider where Sonos wants their bread and butter to be. Is it a networked home music system, or do they want to only be a home theater system company? I would guess they’re following the money.

There’s ups and downs both ways, but i don’t see Sonos making a major shift from where they built their company. 

It does, however, allow you the opportunity to build your own specialist company, that provides this particular feature without needing to include the music aspect of streaming across multiple devices, but only the home theater aspect. Perhaps you could be wildly successful.

You would need to increase the delay for all SONOS Rooms except the PORT driving the SONY. RAM was much more expensive when the SONOS protocol was introduced in 2005 and it is not practical to upgrade the RAM or processor. If current units implemented this sort of feature, they would not be as compatible with older units. SONOS is also attempting to keep setup as simple as possible. Variable delays in each unit would be an unreasonable complication for most users.

If you like complicated, you can stretch out Line-In’s latency. If you introduce another PORT to be the source of all music on SONOS, then connect it’s Line-Out to another Line-In, you have some ability to increase the latency on that second PORT’s Line-In. Likely you would then need to increase the latency on the SONY to align with the new SONOS latency. Operationally, this would be somewhat contorted.

I have no idea about Sonos’s business model.
But I don’t see Sonos offering a proper video compatible surround sound sound system.  So if their main focus is basically about selling hardware, like Apple, then they should accept that they will have to interface with other company’s systems and provide compatibility features; such as AV amp compensation delays. 

Sorry but I think you have missed the point.
I stated that In my neighbour’s house most of the rooms are acoustically isolated. The only problem is between the acoustically connected lounge and kitchen/dinner. Therefore to fix the problem you only need to increase the delay to the kitchen/diner Amp. 
And yes, it is probably impractical to add RAM to existing hardware - unless it is socketed. But newer hardware could have more RAM. Given the price of Sonos kit and RAM the RAM cost is insignificant. 
Your argument that newer units would be incompatible with older units is simply wrong. Adding the feature would simply mean that newer units have the feature and older ones don’t. They would still co-exist on the same network. The older units simply wouldn’t offer the feature. 
If Sonos’s business model is to sell more hardware then providing features that only work on new hardware sounds like an excellent plan. I believe that is why my neighbour has a whole stack of older Sonos kit that he now needs to sell because it is not compatible with the recently acquired stuff. I’m not a Sonos expert, I’m just reporting what I see. 

And as for keeping setup as simple as possible, yes let’s do it for simple systems. But when you have more complex systems then you need an ‘Advanced’ menu. That is why Sonos’s default operating mode is by WiFi. You don’t need to run wires anywhere. And why Sonos sells the Sonos Boost - because sometimes you need a dedicated WiFi network. 
Actually my neighbour’s system is so complex because of the number of systems, walls and other WiFi devices that to get it to run reliably we have had to hard wire it using the Ethernet connections on the back of each unit. Umm, if setup has to be simple then why does Sonos provide hardwired Ethernet ports?

Ahh yes, using the Line Out and Line In connections. These are Analogue signals. The whole point of Sonos, and other modern music delivery systems such as Tidal, Qobuz, Spotify, ROON and Meridian is to keep the signal in the digital domain for as long as possible. Thus avoiding distortion and noise (yes I do know about jitter etc). Feeding a digital signal such as from Tidal into a Sonos device, doing a D/A conversion, feeding that out of an analogue port into a different analogue input, doing an A/D conversion, processing that signal, doing another D/A conversion and then feeding it out to speakers is just perverse. 
And all for the sake of not providing a simple delay function!

My own home’s multi-room HiFi system is based on ROON. I’ve just checked and ROON offers a variable delay of up to 1000ms specific to each individual Endpoint. 

Similar comment could be made with respect to the SONY A/V. Why does it need to add a delay?

The delay is caused by the software processing needed to generate the 5 or 7 surround sound signals from the original stereo. The video signal then has to be delayed to match the sound to get lip synch. 
If you change the processing mode to say Direct then you take out the surround sound software processing and hence the delay changes. To maintain lip sync the amp either takes the delay out of the video path or adds delay back into the audio. Reading the other threads on the topic it seems the more common option is to add audio delay. As the data rate is lower for audio than video I guess that adding audio delay is easier. 

You’re quite right that this issue has been doing the rounds for years. Technically it might be possible to come up with a scheme that could satisfy the small proportion of folks with Connect/Port + AVR and another room grouped audibly nearby.

Why have Sonos not offered a solution? Simple: priorities. There’s a never-ending list of development requests, and it evidently makes zero commercial sense for Sonos to divert precious engineering resource to address the issue, compared to the return on efforts directed elsewhere. 

The Port apparently sells in relatively small quantities compared to the speakers/soundbars. Some probably go to installers feeding third party home-integrated systems, some to HiFi buffs wanting to Sonos-ise their existing mid/higher-end gear, some to those with an AVR. And of that fraction of a fraction of total Sonos sales, a fraction may sometimes want to use Port + AVR with an audible room grouped nearby. It sure sounds like a tough sell for a product manager, compared to all the competing demands. 

If the source is stereo, use the direct/bypass mode on the Sony Amp, to avoid the processing delay on the Sony A/V Amp? 

According to the OP, direct mode makes no difference.  I find that strange. Perhaps to all the fractions correctly identified by @ratty we should add 'the fraction of A/V amps for which the problem cannot be solved by flipping a switch'?