You know, personally, I think the limited number of enemy types (and to be honest, there are actually quite a few of them including variants, and the variants aren't just more health/damage but actually actually have unique patterns and counters - they have more difficult behaviour, which is nice) actually works WELL for the game.
Because the game doesn't get it's challenge and fun from individual enemies, but the way the combine. I've realized why I like playing and don't see it as a grind - it's because it ultimately comes down to my personal skill. Each room is a puzzle, and it's about figuring it the best way to tackle it and then executing that, but the rooms are always different. While the powerups help speed things along, I know that ultimately what's limiting me is my timing and prediction abilities - In all my time playing (and I'm currently on New Game+) I've never felt like I was trapped and backed into corner, and every hit and death (aside from the initial learning ones, of which there are few because there aren't a great number of enemy types) was my fault.
It's not a grind, at least for me, because every go at the castle isn't about getting gold so much as it is improving my own ability to read the enemy patterns, respond appropriately, control my character and keep an eye out for things to look for. About knowing when to fight, and when to say "fuck this room" and go somewhere else or dash through it and just try to get out the other side (and maybe grab the chest on the way). And the way the enemies upgrade, they strictly build on their previous abilities in a way that doesn't put all your previous learning to waste but still provides you with new challenges, and I really like that.
What would more enemies actually DO for you? How would it actually add to the game? Especially since the people calling for them seem totally unable to handle the ones that exist now... I thought the game was actually pretty good at scaling both the enemies and the rewards (which make the previously conquered challenges pretty easy to beat about when you start encountering harder ones) to keep a pretty good challenge level at all times. The only time it ever felt like a grind to me was when I intentionally put off one of the bosses hoping I could buy some upgrades and power through him before realizing that approach was stupid and boring and I was far better off actually figuring out how to beat him - which I did, after a few tries, without taking a single point of damage (thanks to the architect, there was no stats purchased in the meanwhile, and very little in the way of difference among characters - I ended up beating him with a Paladin)
So basically - I think those who aren't enjoying the game have every right to not enjoy the game, but complaining that it is a game about grind says more about the person complaining than it does the game itself, I think.