Everyone should be allowed at the first debate. And then, for a $10 donation viewers should be allowed to vote for which 10 candidates they want in the next debate.
And then at the 2nd debate everybody can pay another $10 to vote again with the top 9 remaining.
And so on and so forth...until it comes down to the final two.
That is why we have polls. RCP averages all polls and it is a good indication of whom the voters consider top 10 candidates.