I agree. The question isn't whether the PC WILL want to learn the ability and do what it takes.
Just posing impossible conditions is exactly like forbidding it with extra steps...
The question is whether or not the PC is capable of learning an ability that there is no in-fiction reason to be unable to learn.
And the problem is that you are assuming that there is no "in-fiction reason to be unable to learn". For example, "
Aura of Murder. As long as the death’s head is not
incapacitated, hostile creatures within 5 feet of it gain vulnerability to piercing damage unless they have resistance or immunity to such damage." You have to be a fanatic of Bhaal, and therefore evil, in a campaign that does not allow evil PCs. Moreover, it's not learned, it a gift from the god.
I don't understand what this has to do with my argument.
Simply that if you have to take a PC path to progression (since they seem to be the only ones available), how do you designe a reasonnably good merchant with expertise in bargaining but no fighting skills at all ?
By 5e RAW some NPCs can. Not all NPCs have no class levels, but this is a Red Herring anyway. Whether or not NPCs gain levels is entirely irrelevant to whether or not a PC should be able to learn a skill that has no in-fiction reason to be barred from the PC.
And again, you are assuming that it's just a skill. But first, it might not be a skill that can be learned, it can be magic that only operates on specific individuals (again, common fantasy trope), and second, once more, are you ready to allow abilities that are incredibly powerful in a PC hand like Parry (+6 to AC as a reaction, absolutely at will).
And again, trading off with something unacceptable to a player does not count, it's just as good as saying no with extra steps.