SP system as it currently is, have no future in RPGs.
Resource as managment system is alright, as long as it is done correctly.
What ToS fail at is:
1)You create game that revolve around skill usage, not around autoattack - meaning you MUST HAVE SP, its not something GOOD TO HAVE
2)You have super small amounts of SP regen and have no ways to regen it in sensible amounts to keep up the pace of the game
3)You must spam potions in order to keep up with the base of the game. Wouldnt be bad concept if the potions were something like in POE, where they regen as you kill. Here its just spam of something that wanish.
There are many resource managment systems that work well, ex:
1)POE - you have potions that regen, you kill and you get them back up, this way you keep the pace of the game. You also have mana leech which allow you to minimize the need of potions. POE is still using the outdated concepts so its not perfect example. The other drawback is that the game is centered around spam of one skill only.
2)Diablo 3 - some skills generate mana, other strongers spend them. This force you to manage your resources, but you dont need any items to keep the fighting at healthy pace. Thats how it was on the release on D3, now its probably spam fest with some builds. The drawback of it - you use only skills that are the best, since you dont want to waste resource on fillers.
3)Wolcen - new rpg that is during development, it use dual resource system. Some skills generate resource X and eat resource Y, while others generate Y and eat X. You must mix skills of various types in order to keep the battle at pace.
4)Many other games - simply remove SP and work with cooldowns which are resource by themself. Give synergy between skills (especially in game like ToS) that will reduce or reset cooldowns of other skills creating some sort of combos.