I think, had Spike not moved over to AtS, he would probably still have had a similar ending in Buffy- it was his heroic moment, the point which exemplifies that rather than doing "the right thing" to be with Buffy, or benefit himself, he is doing the right thing for the sake of doing the right thing. Depending on what your interpretation of the scene is, the Buffy/ Spike relationship is either resolved as "Buffy doesn't love Spike, though he is important enough to her that she tries to tell him that as a reward, but in the end he accepts it and fights the good fight anyway", or "Buffy is finally able to recognise and admit that she loves Spike, even though he doesn't quite believe her". As much as it could do, given that S7 didn't have the best writing, IMO, it tied his storyline up nicely.
I mean, if Spike is not going over to Angel, and Buffy is not continuing, I don't think that there is any reason to not kill Spike, really. If anything, it would make more sense to keep Spike alive, knowing that he was going over to Angel, kill off a different character, and then had Spike and Buffy choose to go their own ways for a while (cue the cookie dough speech again, or similar). I could be wrong, I honestly have no idea if this is the case, but I assumed that the amulet was mostly to give the Scoobies a weapon to call an end to the battle in some way, and to give Angel a reason to show up in the finale, rather than the amulet being written in as a way to move Spike over to Angel, it seems a bit overly complicated IMO. I get that there's this whole "Which vampire will Buffy end up with?" thing going on with the canon (which hurts it more than helps it, IMO), and killing Spike off would have ended that, but since Buffy was ending, and S8 wasn't a certainty at that stage, I wouldn't have thought that keeping it ongoing would matter so much.
So, yeah, my rambly thoughts on the matter.