I recon its more an overall leaderboard problem than top dog spot.
If there's say 25 people with 3 profiles all in the top 100, only 25 others can share those spots with them!
Who knows if people kept to 1 profile each and deleted any secondary ones, I might climb the ranks slightly by default
But, time will continue to tick and people will keep playing, so the top dogs will just better thier own score again and again and it's completely up to you to top them.

I didn't think about any of this occuring in HD, but come to think of it, I have been overtaken by a few, that I know are teams of track creators, including some of the top dogs, so I guess it happens there too.
Not too fussed about this, as its not something a lot of xbox live gold subscribers would be into IMO.
Besides time played is not recorded so noone can look at how many hours it took.