You all are assuming prime weapons will be usable in every content even tho they said they had to create the new content in order for the weapons to have sense.
I smell something similar to the early stage of the relics, when you would get the ws only in Dynamis.
Fujito said:
After "Embryo" is completed, we will be able to enhance prime weapons, but even if we go to the trouble of creating them, they will be meaningless if there is no place to use them. On the other hand, we don't want new content that tests the power of prime weapons to be mandatory for future play, so we are planning several new master trials.
Pretty clear primes aren't exclusively for master trials, but new master trials may require prime power. What's unclear is whether that'll be some content-specific mechanic, or if finished primes are just intended to be more powerful than any current options.
There are def. some players who would still do it, but grinding 180+ hours of Sortie just to unlock some cosmetic challenge mode fights isn't going to be most people's cup of tea.