Is there some sort of question here?

Networks change all the time, from self adjusting where they can, to auto-updates to the router’s firmware, to outside influences causing p wifi interference , even though you made no changes. Networks, even ‘wired’ ones, are not static, by design. 

I'm well aware of how networks work. There is absolutely no reason that I would need to setup the system again. They've had repeated issues with network connectivity in the past, and yes it was Sonos' software update. 

Have you reserved IP addresses for the SONOS players and their controlling phone/pad/computers?

There is no reason to setup the system again. If you check your router you should see all your Sonos devices are still on the network.

The issue is the mobile app: it has real trouble finding those devices, sometimes. For many folks simply force killing the app and running it again is enough to find the devices. For some folks this is not enough, maybe rebooting the router and then the Sonos device will make them show up.

Meantime you can use the Desktop app or third party apps, all of which can find Sonos devices more reliably than the mobile app. But please, don’t be factory resetting any devices, they are fine.