I am not sure what you are referring to in terms of progressively more. The Cavalier does not get additional opportunity attacks until 18th level with Vigilant defender and there is no progressive number.you mean like what the cavalier has they have increased number of opportunity attacks and progressively more as they get higher levels.
The original post suggests being able to use a somewhat similar for the core fighter it is not radical as an option replacing action surge (which makes it worse than taking the Cavalier and completely inferior by endgame but allows one flexibility of still taking a subclass). Arguably the original post enables 3 different approaches without switching to a subclass. Spell casters get many many more choices of that ilk based on their spell selection.
With Vigilant Defender they can theoretically make a number of reactions limited only by the number of creatures in combat, but they still have to be triggered and they still only get one attack per trigger event. It is also not any reaction, but an opportunity attack specifically, so things like the Sentinel or Mage Slayer that allows a reaction attack do not apply because that is not an "opportunity attack". Finally it is a single attack with each opportunity attack, not an attack action with extra attacks and it is limited to one reaction per turn.
The unwavering mark feature uses a bonus action to make the attack, not a reaction, and you have to give up on another bonus action to use it, so it is limited to 1 additional attack per turn and 5 total per day, so that is not a reaction and not an opportunity attack.
I am not sure what you mean. Any reaction needs to be triggered, some abilities give additional ways to trigger OAs, but they do still need to be triggered.Extra opportunity attacks really only makes someone better able to "thou shall not pass" also remember enemies do not have to trigger them.
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