Fascinating... 3.5/5e style multiclassing is the single feature that I like best about these systems. What you see as an incoherent form of point-buy, I see as a perfect middle ground between the endless choices of point-buy and the derth of choices in a single-class system.
One of my biggest frustrations since... about 2006 or so has been fans of the 3.X multiclass system defending it on those very grounds.
While, at the same time, you have to jump through a
ridiculous number of hoops to get certain abilities. So many Prestige Classes, each with their own prerequisites, just to get some fairly basic customization--
why were those abilities not optional features of the base class?
You describe 3.X as the "best of both worlds" between a point-buy system and a class system. I see it, very much, as the worst of both worlds: you have the limited choices of a class system, with the analysis paralysis of a point buy system, with the added burden of having to
qualify for all of the options you're trying for.
I am not advocating for a "single-class" system, by any means-- the last time I willingly
played a single-class character was in 1992. I won't even play 3.X unless it's using some variant of the Gestalt rules. I am just arguing against the specific form of multilclassing in 3.X and 5e, because I consider it the single-worst thing to ever happen to D&D... followed by a long line of other unnecessary threeisms that ruined AD&D.
Fifth Edition was a tremendous improvement over 3.5, but it's still frustrating to see them
revert to so many terrible ideas I thought they'd finally solved in 4e, even if the overall game itself was not my cup of tea.
EDIT: And I am reminded of another of my complaints about 3.5 multiclassing, compared to other combined point-buy/class systems like
Rolemaster and
Player's Option: in those systems, when I select my class(es) and my Kit and my abilities, my build "comes online"
at 1st level. I am playing the character concept in my head from the beginning, instead of having to wait for over half the game to get there.