While uncapped tomes might seem like a good idea from a personal standpoint, being able to grind out full sets of gear within a short period of time.

The game as a whole would likely suffer. Most notably, because a significant portion of the playerbase consists of fair-weather players. You know the ones, they log in only when there's new gear to grind out and once they max out their gear the unsub/just don't log in until the next round of gear gets added (Meanwhile, whining about how stale the game is and how there's so little content)

To which, uncapped gear grind will allow these players to advance to the point where they no longer are active in the game much faster. Thus reduce player interactions across the board (Fewer people running roulettes, raiding, crafting, buying stuff off the MB, farming trials etc) which is a net loss to anyone who participates in this content.

Of course, you could argue that catering to this type of player isn't worth it, given that MMO's thrive off subscription income as well as player loyalty... However, it happens that no small number of MMO players are this type of player, where their gameplay literally just consists of them hopping from gear upgrade to gear upgrade and unsubbing whenever there's no progression left for them.

It's just an inherent design with MMO's. People run content to obtain rewards. Once they have the rewards, they no longer see the reason to keep running content. Essentially, people aim to get BiS so they can have the best stats to unsub with >.>