I get your argument - specially because of the bots - but I dont agree with your suggestion because of two main points (and for the record: I'm not competing here, I'm only after the skybuilder-scrips rewards and a fun expert-recipe challenge for the challenges sake or in other words: I dont have a horse in the race here and am not worried about losing any spots):
1) As far as we know (or at least as far as I know) the reward will be "only" a title - so its not like you're losing out on much, at least not in my opinion. I've heard voices being a bit upset about such a lame reward already but in a sense that almost makes a good one: No one will miss out on something extremly intresting, but people still get their bragging rights.
2) It wouldnt be much of a competion if there was a cap now, would it? You'd also end up woth potentially more than 10 people with the highste score, if everyone is capped at a certain amount - so... would you give out the reward/title to more than 10 people, making it less special and therefor valueable? Would you just draw 10 names out of the pool of highste ranking ones?

They need to figure out a way to prevent those bots competing for sure - and I'll admit that I'm not sure how I feel about people who are spending 18h+ a day in diadem, that really doesnt seem healthy. But I'm not sure if its really up to SE to make sure that people arent "no-living" this (though I get that its frustrating for people who only have limited game-time). Therefor I'm not sure what could be done to combat the "problem" of legtimate people just spending way to much of their freetime on this (...I hope this personal opinion of mine can be forgiven here...)
But limiting the amount of points wont really help solve the problem, it would only create new ones. Sadly I dont have a solution either, though - so maybe take my first point as consolation: You're not missing out on much.