Is there a limit or threshold, is there something to (ultimately) win?

there's no limit! and there's also no requirement. you can play the combat as much or as little as you would like, the community here is much more than just creature fighting. the only limitation is alliances require mp4, but joining one is also not required to communicate with others, they are entirely optional. 

If you enjoy building up your character and creatures, then grinding forever is for you!  You might be forced to move to higher Mind Power levels and contest with stronger opponents, but there is no limit to your growth.

However, there are many other facets of the MD realm.  Role play, puzzles, contests, and quests, to name a few.  If one of those areas attracts you more than combat, you don't need to worry about combat at all.  No worries.