i dont understand your argument why miners have only a limit time? do you mean because the network hashrate can increase?
that argument doesn't revoke anything because the entity with over 51% can just buy/produce more hashpower.
even if i give you the point that a pool might not count as a single entity, because miners could leave if the pool acts in a malicious way -
bitcoin is facing the same problem when an entity decides to buy/produce more then 51% hashpower - tbh even 35% is enough, but u should know it if you read the thousands of articles about it.
no single entity has 51% of the hashing power .. nobody has even 30% of the hashing power of bitcoin so what you are arguing is theoretical.
sure someone could buy 51% percent of the hashing power but it would cost them several hundreds of millions of dollars and as I said it would only be temporary because the difficulty rises exponentially.
even if someone has 51% of the hashing power for an extended period of time.. they cannot control 51% of the coins..
someone quite easily managed to steal 30% of vericoin through very little effort at all .. all they needed to do was buy, borrow or steal 21% more and they would have been in a monopoly position for the production of vericoins for as long as they held onto their coins. they would not need an air conditioned facility, they would not need to keep upgrading their hashing power to stay on top, they would not need to keep paying massive electricity bills. They would only need to run 1 computer and they would be able to produce 51% of all new coins for eternity (if they so wished)
now if someone managed to steal 30% of all bitcoins in existance or even 20% that would be a very huge problem for the bitcoin economy... but someone having 51% hashing power and being able to produce 25 BTC every 10 minutes... that's not really comparable not even in the same ball park.
people who compare 51% PoW with 51% PoS attacks really do not understand the complete difference in scale of how much power each attacker has.
being king for a day is nothing compared to being king forever.
(well at least until the coin gods hard fork you off your throne.. LOLZ)