There are 2 main ways of replenishing TP. Invigorate and Army's Paeon. Not all jobs have access to Invigorate, and not all parties always have a Bard. TP regenerates 60 per GCD, which technically means you never run out of TP. Most moves can be executed with 60 TP, and almost every move can be executed with 120 (2 ticks). A few require 3 ticks worth but I think those jobs have access to Invigorate.
I'm sure any sort of "logic" around TP as a resource centers around melee, since melee attacks typically don't use Mana as a resource. Sure, it would be more fun with unlimited TP, same with unlimited MP. Heck, why don't we just have unlimited HP, CP, and GP for that matter. Who really needs limitations after all?
Just my opinion: I think you are just trying to find something to nitpick about. The fact that your sig says there is nothing to do in the open world is evidence of a negative impression of this game. Negativity usually leads to nitpicking over minutiae.
I personally rarely run out of TP, and even if I do the TP regen is enough to keep attacking albeit at a slightly slower pace. Are you even having TP issues as a PLD? Typically they don't, since your usual rotation is not as TP intensive as, say, a WAR. Thankfully the enmity increases have reduced the need to constantly be spamming attacks to hold hate. In fact, on most mobs after a few combos you can sit completely still until they are around 5% health and never lose hate.
If you're not having TP issues, then there is really nothing to complain about. If you are just curious why TP exists, you are thinking too hard about it, because it's pretty obvious why it exists. At the end of the day, it's not trying to be fancy, it's just a generic system for skill management that doesn't try to do too much, or make things too complicated. Could it be more interesting or more interactive to enhance the game? Sure, lots of things could be. They keep saying they are listening to customer's ideas, so have at it.