apples to oranges example you got there
Agreed but it's what I had at the top of my head.
Tomb raider is a better one. Sold 10 million, square weren't happy. Made a trilody anyway.
What about the rest of the post?
Another apples to oranges example. Both Mass Effect and Tomb Raider are big existing IPs which could easily continue. Days Gone is a brand new IP. Very big difference.
There's also no way that 8 million sales means an 8 million userbase, unless you expect everyone who bought the game to be have loved it enough for a sequel, which they may or may not even have bought at full price.
Maybe a DG2 could have been something big. But that pitch was back in 2019 when commercial and critical reception wasn't incredibly hot. With the game having a 6 year development cycle from a studio whose previous console release was a PSP to PS2 port 9 years prior, it's not hard to see why Sony may not have been very gung ho about jumping into a sequel.
Not like it matters: they're working on a new open world IP so Sony clearly has faith that they can deliver with a clean slate.