It is not the case:
Question
If the only way to upgrade a weapon is to use it, players won’t want to try new weapons because of the marked drop in power they’ll have to put up with for as long as it takes to grind out the upgrades.
Answer
Powerful weapons will still be distributed through stores/Emperor’s Gift. You can start with already high tier weapons to grind the Mastery, similar to what the game proposes at the moment.
Mastery will just open the toolbox to the player for them to experiment.
Mastery doesn’t “gate” a person from acquiring a weapon with a given blessing, it only gates you from freely being able to add it to any weapon of your choosing. The loot system can still allow players to shortcut to a more powerful blessing before they unlock it in the Mastery.
This also loops back around to “wasted” progression (playing now, before the update, or later with a maxed out weapon family ) because you’ll still always earn materials for the shops/consecrating.