It is funny that it is an OPTIONAL rule, but there are those who would call it "denying" them if I choose not to. No one would be denied anything. I like the idea of keeping it as simple as possible, and without them, there are fewer reasons for disagreements or different interpretations of rules. That said, increasing monster HPs is a good option I have not thought of. More options for PCs is certainly good, but how many come with a little bit of a price. Thanks for all the input and ideas!
You are denying fun by banning feats. Feats are fun, and a good DM does not need to ban feats for the sake of balance. Not saying you're a bad DM, but you are the god of your world, you have all the power at your finger tips, you have plenty of ways to challenge your players even if they take the so called OP feats. If you look at Matt Mercer or Matt Colville, they are very generous in terms of empowering the players, whether its magic items or interpretation of rules. Some examples would be: Mercer allows casting 2 spells using bonus action and action in the same turn, and Colville allows a full attack action from the Haste spell (instead of just one weapon attack). Colville mentioned that he knows he is bending the rules, but he lets the players have their fun, because he knows how to get back at their characters later.
I'll give you another example in my session. I had 2 players who took Sharpshooter when I was DMing a one shot. They steamrolled through most of the fights, until they reach the BBEG wizard. Every time they took the -5/+10, they would miss because the Wizard would cast Shield. They soon figured out that they were never going to touch him if they keep using that. The fight was certainly challenging for them, as 2 players were unconscious, but in the end they prevailed. This is me not only allowing feats, but also giving them one free feat, and allowing them to cherry pick one uncommon magic item. I'm not bragging, but just to drive home the point: You have plenty of options when it comes to challenging your players. Let the powergamers powergame, let the players have fun. What are you afraid of? You are the DM, you got this.
I still respect your decision if you chose to ban feats for simplicity, but you shouldn't be afraid of disagreements with your players, as long as you can explain your decisions/interpretations, whatever they may be. Just remember.... FEATS ARE FUN!!