converting older generation products

The only S1 only devices you own are the Play:5s.  All others are S2 compatible.  As far as the split being "solved", the answer is never.  Otherwise they wouldn't have had to do it in the first place.  The fact is, the old devices simply don't have the memory or storage to keep up with the new products.

Thanks. Is there a hack to allow the old gen speakers work on the wifi network, perhaps by creating a new system or using a guest network? They were expensive speakers and they work well - I don’t see a reason to buy new ones!

No.  As stated, if they could do that, Sonos never would have split them off. 

You can easily set up an S1 and an S2 Sonos system on the same WiFi. You will need to use the proper S1/S2 controller to run each system and they will not communicate with each other or play in sync.

Power down all the S2 Sonos and run the S1 controller to get your S1 devices set up.

Power down all the S1 gear and use the S2 controller to set up your S2 devices.

Once set up each controller should have no problems controlling the connected devices.

https://support.sonos.com/en-us/article/known-limitations-with-separate-s1-and-s2-sonos-systems

https://support.sonos.com/en-us/topic/sonos-s2-and-s1