Yes if you want your PC to retire from adventuring and spend the next 5 years training sure go ahead, in the meantime roll up a new character and let's continue the current campaign we are in. Or find or create a homebrew class/subclass/feat for whatever it is and if it's balanced it will be allowed.
And maybe it's just me but most of my adventurer-style NPCs are going to adhere much closer to the PC rules, especially the iconic class/subclass feature. If I was making a fighter for a rival adventuring group I probably would give them Action Surge, Second Wind, Weapon Masteries, etc... and not use the Gladiator stat block.
What's ironic about the whole if an NPC can do something the PC needs to have a way of doing the exact same thing is that it encourages providing Trap-options for PCs. As I mentioned earlier in the thread you can very easily match the HP bloat of these type of NPCs by allowing the Tough Feat to be taken multiple times, but frankly that's a trap option if made available to PCs. But for NPCs giving them trap options will often make sense.