The system shouldn't be made more complicated just for the sake of being more complicated.
If experience eventually shows that there are some additional privileges that would make sense to grant, but only for users with super high reputation, then they could be added. To the best of my knowledge, it's not particularly important that the highest level of privilege starts at 20k. That's just how it currently works.
But it's how it will likely work for some time to come. New privileges shouldn't be added just for the sake of doing so.
Some powers are especially dangerous for any small group of people to possess exclusively. There are three general solutions to that problem:
Let no one have the dangerous ability.
For example, anyone can edit or propose edits, but no one can modify the contents of an entry in a post's edit history, to make it appear like someone else wrote something different from what they did write. The problems with such an ability far outweigh any benefits. (Stack Exchange employees can remove edits from a post's edit history, but they do not rewrite them.)
Let everyone--or at least a large number of people--share the dangerous ability, as equally as possible.
For example, voting on posts could be considered dangerous, in that people often rely on votes to figure out what posts are worth reading, and if many people voted wrong, it would be bad. But voting is shared by many people, instead of being the responsibility of any single individual.
So please vote on questions and answers, to help the community!
Let a small group have the dangerous ability, but try to create a robust system to make it easier for them to do the right thing and harder for them to do the wrong thing.
For example, some actions on Ask Ubuntu can only be done by moderators, but they are elected, expected to obey special rules, considered to be especially accountable to the community, and the Stack Exchange Community Managers moderate the moderators.
I think giving a tiny minority of individual users special extreme powers who have not agreed to the elevated responsibilities associated with them is not a good idea.
- The current system gives special powers to high rep users--we can vote to delete answers, for example--but we don't get to delete other people's accounts or single-handedly delete a post.
- Such a dangerous power should only be in the hands of someone who has specifically agreed to accept the associated responsibility, and who has been selected for that role by the community as a whole.
Please note that I am not saying every possible community has to have moderators. (Many forms of organization and governance are possible, both offline and online.) Only that if you effectively create a small group of moderators, it's good to explicitly call them moderators and expect them to behave as such. Which is what we do.