Players should be able to respec their Mastery class at a high cost and here’s why:
There is no way at level 25 that you can make an informed decision about what Mastery you should choose no matter how good the description is.
It forces people to do a bunch of research about what they should choose online (which they shouldn’t have to do) in order to play the game. You also don’t know what Mastery is going to resonate with you as a player.
Towards the end of the game so much of what you have to do is experimenting with gear and different builds that you are essentially punishing players and limiting their access to do that.
The vast majority of players will be “casual” and will not have the time to reinvest in leveling a duplicative class just to choose a different Mastery. At this point there is a pretty good chance that these players will lose interest in the game and move on. For the elitests out there I’m sure this is fine but let me appeal to your business sense because after all, this is a product and you want to retain your customers.
The wise decision as a business and a game developer is to make this game have the widest appeal possible. The best chance for success is to allow players to experiment and find a class/mastery they fall in love with and are willing to sink years into playing. That is why you should allow players to respec their Mastery at a high cost.