Making a decsion like that for a brand new and untested IP that was on a single platform would be far too much of a risk for any company, you only have to look at Days Gone to see the risk of assuming that something like this is going to be a major success. It's different with things like Assassin's Creed where they pretty much know how many sales they're going to get before it's even launched.
It is worth remembering that for Sucker Punch it has actually been standard practice for them to release expansions to their better selling games since 2009. Infamous 2 got Festival of Blood, Infamous: Second Son got First Light and now this is as a multiple award winner and the fastest selling new IP on PS4 is getting it's own expansion.
I can honestly expansions like this being far more of a thing moving forwards now that Sony are wanting to expanding all studios to have more than one team and more than one game in production at at time if a game has been as succesful as Tsushima was.
IMHO the "risk" isn't really a factor in the wider picture, because it isn't sucker punch, they are owned by Sony, and Sony need to put out these games - even if unsuccessful, as they have done for 25years, to sell the hardware - the risk is already in the trying to sell a console platform, so it is just opportunistic cash grabbing IMO, and they might even use the meta sales data from the Expandalone - hate that shakedown terminology - as an excuse to not do a GoT2 and finish the story.
Funny you mention Infamous, expandalones, as that was exactly the time when I stopped with that IP - after finishing Infamous 1 & 2.
IMHO, you have optional DLC or you have sequels, Expandalones are an affront on the trust the consumer puts in the developer when investing in an IP, as they so far have always been content that has been held back from the main development. If they launched on disc and were actually developed from scratch after the release of the game they expand, they I would look at it less harshly, but as is, IMO it is cynical and long term calculated shakedown of the players.