I read an article on gamepress, and the part that interests is: Origin Pulse and Precipice Blades are already coded.
Even if we only get a legendary with 90% IVs, its still fun to power it up, because we know its a mighty pokemon. But what happens when we know this mighty pokemon gets a mightier 2.0 version during a 3hour event window? Why power your old one anyway?
Every Legendary has a signature move, so if Niantic releases it for the first time on a event, we can be sure that it will happen to every other Legendary. On Gen IV we can just hold a few Dialgas and Palkias until they get a more powerful move.
Go Hub already had an article on this issue. I too think its a dumb problem. Why not let us TM our old pokemons during an event? I think it seriously discourage a player to invest hard earned stardust on something that will definitely be better on the future.