Note that sealed AGM (Absorbed Glass Mat) batteries depend on letting the pressure rise to promote a reaction at one of the plates that turns the gasses back into water. (They have a valve to relieve excess pressure from overcharging TOO much - and that's the mechanism for water loss.)
Yes if they dry out you can refill them if you successfully open them. But you have to reseal them airtight or they end up acting like a flooded cell that's almost out of water - running dry again at the first hint of an overcharge. And you have to be careful that you don't damage the valve (in a way that would keep it from releiving pressure) when you reseal it or you might end up with a bomb.
If you don't overcharge them into drying out: the other thing that kills them is when the spongifying of one of the posts (a side-effect of the charge/discharge cycle) makes it expand and rise out of the case. There are O-rings to allow the post to grow up without breaking the seal. When the last O-ring is about to unseat it's time to send the battery off to be melted down.