Zamasu is my least favourite DB villain. Here is an explanation why.

[SupremeKai25]

What was going through the writers' head when they decided to make Jiren and not Fused Zamasu the final villain of Super? Fused Zamasu had all the traits that define a final villain. A superb design, unique and flashy techniques, a deadly gimmick, and interesting motivations. But apparently Toei and Toriyama decided that Jiren, and not Fused Zamasu, is "final boss" material.

[PerhapsTheOtherOne]

What was going through the writers' head when they decided to make Jiren and not Fused Zamasu the final villain of Super? Fused Zamasu had all the traits that define a final villain. A superb design, unique and flashy techniques, a deadly gimmick, and interesting motivations. But apparently Toei and Toriyama decided that Jiren, and not Fused Zamasu, is "final boss" material.

I think I might have an answer.

Zamasu represents the peak of "nuanced character building", at least for the franchise's roster of villains. Jiren, however, represents the slippery slope of the franchise; he's a final antagonist who's nothing BUT raw overwhelming strength. He's the personification of the community's obsession with a character's power level above all their other traits.

I felt that he was a good deconstruction and parody of that obsession, and the reactions of people to his "bland stoicness" show the hypocrisy of that mentality. We also see how such an obsession would play out for a character; he's broken and can't form proper relationships, and is reliant on his strength as a crutch for his past failures.

It really couldn't have been anyone BUT Jiren, the guy who was simply the strongest there was. Because in the end, that's what the franchise fell into: how strong the next antagonist was, and how that strength needs to be overcome.