Blue
Ravenous Bugblatter Beast of Traal
...
Sorry, the last campaign I was in I was playing a bard. Expertise in persuasion, was the party face.
New player came in months later, played a multiclassed rogue (Arcane Trickster) / Fighter / Cleric (Forge) with a high CHR and expertise in persuasion. Same modifier as my existing character.
(Side note: I tried to talk to them, that it was my thing already in the party but I wasn't into deception or intimidation and we could be complement each other. No luck - his concept was "merchant", he needed it to be persuasion. Not sure if Arcane Trickster / Cleric of the Forge would be what I would pick for merchant, but whatever.)
So, let's assume that between AL games where you don't know who you'll be playing with, and with players who are self-absorbed with their concept, we can't assume that there are gentleperson rules of etiquette protecting niches.
Anyway, moving back to the main thrust. It's the specilality of some classes to be skills monkeys who are really good at some things without magic. If any Tom, Dick, or Fighter-with-extra-feats can also do it, it dilutes what makes those classes special.
In addition, at higher levels when both (a) your first choices for ASI/feats are already fulfilled and (b) double adding proficiency makes a bigger difference, I see this as drastically changing the flavor of a character. "Oh, I know you were the best at X for the whole campaign, but I just added +4 and surpassed you. Next level at 13th it'll go up by 2 more."
