It does get into a grindfest, yep. That's why I lost interest in it in the first place.
Now I’m back in and was surprised to see that they have new content. I'm sure after a while I'll lose interest again, but I've gone from level 40 to 70. The PvP tournaments are interesting. I didn't think I'd like them, but I find it an interesting challenge to crack open some of these defenses. Sadly I think my defense sucks, but such is life.
Some of my friends apparently haven't gotten tired of it. Their levels are 3 digits. I'm not sure they ever stopped playing since it came out.
But the grinding really didn't stop. Some things are a bit more interesting (for example, Ultron is a group boss that can be defeated by you and your friends), but it's still grinding with the means of advancing your team so that you can grind faster.