Why vertical progression is poorly implemented

[Sylkis]

"I felt a great disturbance in the Echo, as if millions of retainers suddenly cried out in terror, and were suddenly silenced."

There would be only 20 shields (minus the ones used in leveling up) in the whole game up until now, as an example. So 20 (endgame) shields since 2013. And most importantly, you can't have them all but you have some to choose from (unless you are someone who actually farms relics for all jobs)