Regardless of personal opinions on the ending of ST, Fear Street has been done for years and the time for that passed. Prom Queen (from what I heard) was pretty universally disliked.
If we were to look at this from a general public/crowd demand and financial aspect, ST makes a LOT more sense. The merch prints money and it's the last shot to have that at the event where it's still considered timely. A much larger crowd watched the finale than will ever watch Fear Street (despite the original trilogy being popular regardless). Plus, to have all the other ST seasons and not 5 would be weird.
Again, this is taking into consideration that ST isn't actually even an official rumor. Fear Street is even less so.
As for my personal opinion that no one asked for lol:
I don't think the end of ST was THAT bad. I overall found it to be a satisfying conclusion for most of the characters, and I'm even ok with the open-endedness near the very end. I just think there were a lot of details that could've been tweaked or addressed for a more impactful ending. The first half of season 5 was where I had more issues, and it was primarily in pacing and the writing of the dialogue, but that's also coming from my bias of being a writer with a love of dialogue lol
Tl;Dr: It was aight.