Well, it also matters that the set be constrained to some degree.
A large flat root would not be very managable, and caches wouldn't
be very effective with large numbers of TLDs.
That's old fiction. If it works for .com it will work for ".".
well, it's not clear that it works well for .com. try measuring
delay and reliability of queries for a large number of samples
sometime, and also cache effectiveness.
let's put it another way. under the current organization if .com breaks
the other TLDs will still work. if we break the root, everything fails.