Like the post above clearly shows, it just does not work like that for the balance side of the game. Plus a bigger hero-pool itself brings it's own sort of balance. After that it's just a matter of everyone having their favorite heroes.
The thing is, if you picked out 30 heroes, the metagame would shift heavily. I'm not sure it's fair to judge 30 heroes from the perspective of the current metagame when alone it would be completely different. For instance, a hero that might be balanced now is so because of several counters, which may not be present in a 30 hero pool.