In theory, I like the idea. I think it's a step forward for games (and RPGs in particular) to allow choice between a fixed protagonist and a custom one. I personally like custom PCs more, but I enjoy games with different approaches and I think they all are valid (fixed Geralt, Grey Warden origins, custom Dragonborn).
In reality... I don't hate origins in BG3, but there are several problems with them.
1. As Tuco said: resources. Lots and lots of resources. That could be better used for something else.
2. I don't know if that's still up-to-date, but Larian stated that they want all companions to be origins. They also want each origin/companion to be extremely story-rich. I'm normally for quality over quantity, but that's too much "quality" that came at the steep cost of the choice of companions being... poor. Very poor. Half the BG2 companions, more or less.
3. Origins are not fixed enough imo. We already have custom protagonists if someone wants freedom of roleplaying. Larian decided to not limit our dialogue options when playing origins (afaik), which makes them... diluted? I think if you go with a fixed character, it's a good idea to trade some player freedom for more personality. (I'm not saying remove all options, of course, but limit those to what the character could realistically say/decide. There are still multiple origin paths to follow.)
4. You have to tailor the story to this dual system. The story has to work with all companions potentially being protagonists. That's limiting the narrative.
5. This is a problem that I think is more a matter of perception than an actual problem, but origins make custom characters look even more bland. I think most people would be fine with not much background for customs (a'la TES), but when you juxtapose those with origins it suddenly looks like there's nothing interesting about custom PCs. Custom characters have main plot, origins have main plot + origin plot. The former seem inferior narrative-wise.
Imo there's a couple of ways to deal with most of these problems, but that would require quite a lot of resources (so sans 1.).