My theory is mayuri dies cuz uriu is a main factor in this arc so he has to get revenge for his grandpop cuz weall know ichigo will fight bach when mayuri dies thiers a open seat and since old man yamma or central 46 isn't thier anymore thier in no reason he and yoroichi can't return to former glory Its wishful thinking but its plausable right
Mayuri isn't stopping Urahara from returning to glory. Mayuri's existence as a captain would seem to have little effect on whether or not Urahara took up the position again. After the SS arc there were open spots that I've little doubt that SS wouldn't have objected to these two taking up, however, Yoruichi and Urahara had other priorities. If there are open seats, and either of these two want them, I can't imagine anyone stopping them. They're experienced and powerful captains, there isn't anyone that would likely be higher than them on a list of potential replacements.