The thing is, Umbrella was more than just a corporation, Spencer had plans of world domination, it doesn't make sense that he wouldn't have had some sort of plan B and would be someone worth devoting a whole game to take down, instead they side step it completely in 4 and he's a pretty throwaway character in 5, it's made all the more worse that they were creating a loose trilogy of games revolving around the founders of Umbrella, the first was CV, the second was 0 and 4 was supposed to be the third.
The way CV deep dives into the history of the Ashford family was fascinating, but they were meant to be the lowest rung on the ladder, you're telling me a game delving into Spencer's whole family history is not worth doing? This is the guy that is the whole story reason why any of this is happening in the first place, who was he and where did he come from? Instead everything is kept very vague and throwaway, I just don't get it.
The series forever lost something when it bungled the handling of the Umbrella storyline like it did, because not only were we left hanging in a bad way, the series went off in a bunch of different directions and things got too complicated lore wise, it was much better when everything revolved around Umbrella instead of a bunch of different groups and organizations, RE4 Remake was an opportunity to correct this but I guess Capcom doesn't want to stray too far into alternate timelines and shit.
The series got better but it's still past it's peak, 7 and 8 took the approach of keeping everything vague and mysterious, what I hope is that Capcom will eventually be able to tie everything together more neatly, but they keep kicking the can down the road, I was pretty disappointed how few reveals there were in 8 with regards to the wider lore.
But at any rate, they seem to be pointing towards there being a bigger picture beyond Umbrella, Miranda definitely seems like she has some ties to this and they could easily retcon Salazar into having ties as well, who knows what the future holds.