Unfortunately, the Beam is the distributor in that setup and cannot be disabled as easily as the surrounds/Sub.  It’s important to understand what is at work here.  When adding surrounds/Subs, in order to maintain sync with the video, the radios are reconfigured to a low latency, one-way dedicated 5 GHz connection from the soundbar to the surrounds and Subs.  This requires a series of handshakes to get right, which is why adding surrounds/Subs takes a bit of time.  The reverse happens when the surrounds/Subs are removed.  So therefore, a quick toggle between the two is not possible.  They could place a hot button that does the add/remove at the touch of the button, but that is not going to speed up the actual process, and doesn’t save much from the system menu commands we have today.

An alternative would be allowing one to mute the front device and just play out of the surrounds, which would appear to be easier to do, but I defer to the actual engineers on the practicality of the idea, especially with regards to a spatial audio setup with Era 300s.   I’m sure switching from sending just surround info to sending a full Atmos audio stream through the Era 300 surrounds from the soundbar isn’t trivial, and given hardware limitations of the decoder and the way it is wired to the soundbar, may not even be possible.  I know you aren’t asking about this, but Sonos certainly needs to consider it when discussing this request.

The beam could remain the distributor without producing sound. That seem very easy to program.

I've been a software engineer for 30+ years.  If I had a nickel for every time I heard something seems easy to program from someone who knows nothing about programming, I'd be a very rich man.  Add another nickel for the times that guess was completely wrong and I'd easily double that money. 

That’s funny, I have a software engineer bachelor degree. but I only have 25 year experience.

It’s not only the programming. Sonos would probably also have have to cater for people wanting to use their Sub if they own one - or people not wanting to use the Sub because it is in the wrong place for the Five’s. Maybe Trueplay does not work well if done for the whole set up when you are only using part of it. And, maybe most importantly, it would ad more complexity to an app that is already to complex for some to handle. 

Then you should obviously know better.  So go back and read my second paragraph.  Carefully.