At first I thought quality would stay the same, only Cuban cigars would become more scarce worldwide (though more common in the US).
Then I realized Cubatabaco/Altadis is a big greedy company and will try to ramp up production.
Quality will fall, no doubt, across the board, though some varieties will be able to keep their value and some new varieties will be created.
Also depending on how open Cuba's economy becomes, if they go to a freer market some of the master rollers may go back to Cuba and some really sterling sticks would be produced.
However if the question is, "ceterus paribus" (all other things held constant) then yes quality can only go down.
IMHO