Sentries are actually extremely strong already but they need to be easier to use imo. The main reason people call them useless is because they don’t really fit with many playstyles so a fix for that would probably be best instead of just buffing them. With how outrageous they can be, idk if more sentry specific buffs will actually solve the problem with them.
It's hard to say with sentry summons. They are
really good at what they are designed to do, which is locking down an enemy spawn point while keeping control over where the spawn point is, but most people only really care about bosses and in those cases they turn into just another minion slot outside of overly specific setups like spawn-deleting The Destroyer and it's much easier to kite a boss over a wide range than it is to maintain keeping that boss in one small area to make the most of your sentries
Adding more Sentry slots does one of two things:
- Makes it easier to cover a wider area with a similar modest amount of extra damage
- Adds more overlap zones where multiple sentries can hit a single target for above-average damage
- Locks down a small area with high damage
Half the time in farm scenarios, sentries have similar performance to just sitting in a box and letting Blade Staff minions peek out of the wall and aggro things that walk up to you, up until you are suddenly able to spawnkill things with them
It wouldn't hurt to have more sentry slots due to their nature, but I'm not sure how much it would actually help in practical terms since it's hard to really quantify how much help a single sentry gives outside of cases like OoA where you go from having maybe two sentries to your name to having 20. They'd probably benefit a lot more from staying active longer than 10 minutes per cast since forgetting to refresh them causes way more problems than your max sentry count.
That said, I am always down to clown with the theoretical thought of having a higher max count of Explosive Trap and Rainbow Crystal summons.