Your current Express can continue to serve the purpose.. perhaps for many years to come if relatively new. Buy some new ones now so you have a stock of them.. support will continue for several years even when sales cease.
There is nothing else on the market that fits the Express role.
There are costly substitutes and more of these will doubtless become available.
Whether another manufacturer will step into Apple's shoes and provide a relatively cheap unit is open to question. Some of the system would be under license I expect.
You will need to buy speakers that have inbuilt airplay.
Or you need to swap to different system.. like Sonos.
Or receivers with inbuilt airplay that can send signal to multiple speakers.
Apple maybe is hoping you will buy many many home pods.
HomePod - Apple (AU)