With how technology progresses, I think the "peak" performance for a console will remain in the ballpark between 100-120 Million units before it's replaced by the new generation model. Beyond those numbers is an abnormality that relies on hitting a mass market, cheap pricepoint that wouldn't realistically be long-term sustainable for a business model that the three platform holders go for. A brand new PS4 is still $299/£250 a full 7 years after launch. In comparison, you can buy a Switch Lite for $199/£179 merely 3 and a half years after the launch of the original Switch.
Granted, you can get some really sweet deals for a PS4 with games included, but you get the picture.
Sony could probably have kept the PS4 on the market for another couple years with a substantial pricecut to $250 or $199 and the introduction of a digital only model. Pushing lifetime numbers to what, 125? 130 Million?
Sony don't really need to "expand" that userbase. PS4 profits have shown that they can remain extremely successful while keeping numbers in the similar park as previous generation leaders. If they at least match the PS4, they're almost guaranteed to be more profitable and more successful with the continued growth of digital sales and PlayStation Plus. They'll eventually introduce the "Pro" model for PS5, the console itself is gigantic so you can absolutely expect a Slimline remodel at some point and Sony's own 1st party production is currently as strong as it's ever been while continually expanding.
The aim should be retaining the current userbase before wanting to expand. The last they they need is a PS3 situation where they let the competition eat into the market.