It seems simple: if you don't want to adhere to a vow, for example, don't play a paladin.
If the limitations of the oath make sense to the player, sure.
Though having one class that works that way amongst a dozen classes that don’t is bad design IMO.
And happily, the Paladin doesn’t work that way. Easy as that!
I get the preference for more focused designs but disagree with the implication that the fighter is not an actual class concept and has no degree of identity. I think armed combat specialist is a fine concept and identity for a character.
“Armed combat specialist” is a class concept!?
How? What is the actual concept? How is it distinct from any other weapon using class? Thematically, the fighter is a class group, at best. It’s the kernel of a concept that is shared by…most classes.
Now, boosting the “epic mundane” stuff like has been discussed so much lately gives the class more identity IMO, but it’s still a class that gets by far most of its flavor from the subclass, but most of its power from the fairly flavorless base class features.
So if I were on a team restructuring classes, I’d split fighter up before I’d put any existing class inside the fighter’s umbrella. Give Ranger and Barbarian Second Wind, give Rogue and Monk Action Surge, make additional extra attacks a level 10+ feat that requires Extra Attack, give Indomitable to Paladin, and off the top of my head I can’t think anything else unique to the fighter base class.
That, or make those core features work like expertise and be available to several classes not just one or two, and split the mundane fighter into
Knight,
Duelist, and
Archer, put brute type subclasses in Barbarian, share space between the fighter classes and the Monk who is acts a lot like a
mystical, but not necessarily magical as such, warrior, and make limited forms of those core “warrior” abilities available as feats.