They offer AirPlay in the Appletv 4 and many new receivers have it, so I doubt the actual tech is going anywhere, even if they have given up on the Airport Express (don't really have a feel of whether they want to move this tech along, but in my case, it already works like a charm. I also have a combination of old and new Airport Express units. The one I use every day is the old one with the huge plug)You are probably right - the issues o have a are likely due to the inherent nature of wifi limitations. Unlike your setup, my airport expresses are not wired. I have four of them, and they represent three generations of Airport Expresses.
Some of the issues - Airplay suddenly can't find one or more, sudden unexplained breaks in Airplay even though the source is not moving around, and certain songs - yes the songs themselves - refuse to play on Airplay. That last one is weird. No idea what it's about.
But I can't say either way if Sonos is better. But I can say that I think the writing is on the wall - I think Apple is getting out of this market - the Airport market. Hope I'm wrong.
The tech in wifi routers is getting more sophisticated all the time, with many new wifi routers coming with their own mesh systems. Personally, I have a TP Link Google Hub and it never has an Airplay drop out. I can even get a strong signal outside. However, TP Link has even better routers out now with multiple lanes and numerous large antennas. I won't be updating my router until something breaks, though. But if I was having Airplay drop outs, I would upgrade or use a Home Plug solution when possible. The upside to just fixing the problem is that your entire network of devices should benefit.