...on the same network as their new speakers?
There is currently only one network. All speakers work on that single network. New speakers still work on the same network.
As I understand, and I am not a Sonos owner, the old and new speakers can co-exist, but not integrate into one seamless system.
Sort-of. They can still integrate into one seamless system but, in order to do so, the user must accept that they can never ever receive software updates for all of their other speakers ever again.
If the user wants to update the software for their other speakers (which could be to fix a bug or patch a security issue), then those legacy speakers must be moved onto a separate network where they can only be grouped/paired with other speakers in that same network. There is a rumour that you might even need to use a different mobile app to control them - although that just sounds bonkers to me.
The introduction of the new speakers in no way reduced the functionality of the old speakers.
This is technically true, but the selling point of Sonos was that it was a whole home audio solution that could be built up over time. In addition, the introduction of additional newer speakers to your home (which is what Sonos want to you to do) causes the user to have to decide which trade-off they want to have.
My point is that this is a needlessly complicated solution. A much simpler way would have been for Sonos to say "In May 2020, some speakers will no longer receive updates. You can continue to use them in your network to listen to music but they won't be able to take advantage of any newer features we introduce after then". This is precisely how they work today with things such as AirPlay - if your device doesn't support it, it falls back to being a basic wireless speaker.
This link provides a bit more details.