If 10,000 votes is easy to reach by bots or organised trolls, why is 50,000 not?

Presumably LEGO will raise the limit if they get swamped, but 10,000 is just the bar to get considered by the staff, it confers no guarantee of success and hence isn't super worth gaming.

I think Lego wouldn’t need to spend too many resources on throwing out 4Chan ideas

10,000 just gets Lego to review it. They don't have to approve it.