I have lots of bitcoins. Your claim that when price reaches desired level, those who have lots of bitcoins will sell for fiat, is wrong. If you have maybe few thousand dollars, and are just speculating, then yes, you can sell for fiat. But if you have hundreds of thousands, or millions of dollars of bitcoins, selling means paying taxes, and telling other people that you have so much money. It also means having your money stuck in a bank some where. At some point, it just becomes impossible to sell it all, because you can not live buying expensive cars and houses every week without being noticed. So you have to keep bitcoins in bitcoins, and only sell whatever needed to live, and maybe use some to invest in various projects.
Sounds like a good problem to have. Can't say I have it yet.
I have pondered this though. There may come a point in the future where it does become a problem managing fiat. Of course we have already thought of the tax implications of selling, but then there is the problem of what to do with the money when it is cashed out as well. It will be wise to keep some in coins for sure.
I say this is not the way to go because of several factors, but just note the likely effects of on line gambling, and of on line shopping, on the fixed real estate models over the medium term...say 10-20 years.
Nonetheless, diversification is a good thing, as an example look up theory of why and how a 'diversified stock portfolio' is widely understood to be a good thing.