It could be possible for all Pokemon to get Mega Evolutions, but it's unlikely.
As of now, only fully evolved Pokemon can have Mega Evolutions, meaning that Pokemon that can evolve probably won't get Mega Evolutions.
There are also Pokemon with alternate formes, these Pokemon will probably not get Mega Evolutions because they already have different formes.
It would be a pain in the butt to collect EVERY SINGLE MEGA STONE. If every singe Pokemon gets a Mega Evolution, that's (As of now) 721 Mega Stones to collect. If every single fully evolved Pokemon gets a Mega Evolution, that would still be a lot of Mega Stones to go around the region to collect.
There are also some Pokemon that are totally useless, so a Mega Evolution of the would never be used, take for example Unown. Unown can only learn one move, Hidden Power. Who would use want to use a Pokemon like Unown if it only has one move? Even if it did end up getting a Mega Evolution.
There are more reasons that this will probably not happen, but these were just a few,
Also keep in mind that this could happen, it just probably won't.
But, nobody knows if this will happen, this is just a theory.