My system comprises an all British set of separates, including a pre-amp and mono blocs. It still sounds great after twenty years, but things change and technology marches on.
Another angle on this: I don't know what make these separates are, but I know of one make - Quad - that is brilliant in supporting their kit for decades. Their UK service set up in particular, is excellent and can run a check on all electronics ever made by Quad and bring it back to As New, with even a limited guarantee. I have used this service from as far as India, and I know people from EU that also have nothing but praise for Quad service.
The reason I say this is that from a sound quality perspective, there has been a lot less audible progress than one might think. Depending on what and how good the passive speakers are, of course. Where Sonos has added features is in making a lot more music easier to access and to play. There may therefore be a case to keep this set up, and add just a Sonos front end in the form of a Connect. You could also add the cheaper Chromecast Audio, but having used both I think the easier to use Connect - even just the few control buttons that the Connect has - easily justifies the higher price when use over a few years is taken into account.
As I said, just another point of view to take into account.
