Think about it logically. What do we gain from unlocking new abilities past level 50?
1. The joy of unlocking new abilities as the level number goes up.
This joy is only ever experienced once per job.
What problems does this cause?
1. Being forced to play an incomplete, dumbed-down version of our job every day in roulettes.
2. At lower levels, losing basic quality-of-life improvements which were only ever gated behind higher levels so that each new expansion could have 5 staggered ability unlocks.
To put it another way, the joy of unlocking Umbral Soul at lvl 76 is nothing compared to the pain of not having Umbral Soul before lvl 76. So why don't we just get Umbral Soul by level 50?
Really, why not? Nothing meaningful in this game is gated behind level. You complete the story to unlock more story (and gain enough EXP as you go to make level-gating irrelevant). You complete the story to unlock side content. You raise your ilvl and beat progressively harder fights to unlock even harder fights. So why is the completion of our basic job rotation tied behind level progression? The only reason I can see is to have something to look forward to as we level.
Does that justify the pain and hardship of not having TBN until level 70? Of not having Verraise in cursed Dun Scaith runs? Of having to play Black Mage without Fire IV, or without Umbral Soul, or without Triple Cast, or without...
Nay, I say. Let us have two charges of our gap closer in UCOB. Let Dark Knight have an AoE rotation before level 72. Let Scholar Expediently rush us even faster through the first pulls of Praetorium. Throw wide the gates, and give us all of our job abilities by level 50.