Glover's reaction was part of it. But the other big thing was Obama's election. Marvel felt that if America could accept a black President, then it was time for a black Spider-Man, and felt that would get them first on the ground floor (which it did) to put out the first major Affirmative Action Legacy and there's nothing quite like "Spider-Man is Black" to grab headlines and news (which it got). It got them over DC in a big way (especially since DC under Geoff Johns was shooting itself in the shoot by throwing away the amazing buildup Green Lantern John Stewart got from the cartoons).
It's not true that Ultimate Peter's death didn't have any lead-up. It was a 7 issue Story Arc. The Death of Spider-Man arc also overlapped with some stuff that happened in the New Ultimates comics. So it was a co-ordinated event with a lot of planning and consideration, planned more than a year in advance. Bendis said he came up with Miles Morales first and then decided to kill off Ultimate Peter. He said that he realized that if you had to think of a working class New York teenagesuperhero in the 21st Century, then it's quite likely that character would be black. So Miles Morales wasn't last minute. Bendis came up with Miles and then decided to start wrapping up a series of subplots in Ultimate Spider-Man. The stuff about Ultimate Peter becoming the big hero, Jonah offering to sponsor Peter, Ultimate Peter and Ultimate MJ getting back after he broke off with Ultimate Gwen...all that was done to set up a tragedy, to give Peter a series of wins so that when he dies it feels that much more painful because both readers and cast get a sense of what was lost. Is that manipulative? Yes. Did it work? It did.
The death of Ultimate Peter bothered me and affected me at the time...I had quit reading ASM after OMD, but I still read USM. I felt bad about it, but I accepted it later and I liked Miles right away.
Looking back, the thing about Ultimate Peter is that he was a version of Peter who was never intended to grow up. he was never going to actually graduate high school and become the fully mature version of Spider-Man from the comics. That was editorially mandated. By the time Bendis was writing him, Post-Ultimatum, there really wasn't too many places you could take that version of Peter and his cast. I've read every issue of Ultimate Spider-Man, both Peter and Miles, Pre-SW'2015, so I know where I speak. Killing Peter and making him the equivalent of the teenage rock god who died too young wasn't too bad. (And anyway, Bendis brought him back later aged up and gave him a happy ending). Ultimate Peter was a great version of the character, and what made him work (and I am not sure Bendis intended this) was that he came off as more mature and saner than the many "adults" like Ultimate Fury, Ultimate Cap, Ultimate Tony and others. That was more because of how Millar concieved his characters than anything.