Again, the different forms of MC can exist side by side in the rules for the GM to choose from what is allowed & what is not if they are actually "optional". WRT your "punish" comment though, who decides if an "optional" rule is allowed in a game, the gm or a player who wants to use it? If it's the GM then allowing a more restrictive version of multiclassing rather than simply saying "no MC is not allowed" then how is anyone being "punished" when restrictive MC is less restrictive than no MC? If it's the player then is the "optional" rule optional at all?What?
This is not what I said...
I said: why do you want to restrict a perfectly fine rule by adding needlessly punishing rules. PB was deliberatley dependend on total character level to allow freedom of multiclassing.
Why are you so mad that people don't do even splits. Is your play rstricted by people who just use multiclassing as is?
3.x/5e style multiclassing does create problems for the gm where Alice finds a build that with a small dip allows far greater power than bob who is playing one of the classes Alice is using. Now Bob feels pressured to do the same & the GM needs to somehow handle both players on very different power scales but needs to do so in a way that won't be problematic if bob takes his new toys & does what Alice did in a level or two.
With a more restrictive version like is being discussed both Alice & Bob have very different needs & Bob can't take the fancy treasure he got to instantly do what Alice did. If Cindy wants to do a 3.x/5e style MC build for flavor rather than power she can talk to the GM & make her case for an exception by explaining why xyz is more flavor than power or how it's just filling a need the group has. In a case like Cindy's the GM even has a strong case for pulling cindy aside for a talk if it turns out that N levels later there's a big problem that wasn't noticed or discussed.