The new system and the old system. Constrained.

[Physic]

But they entire battle system is being overhauled so what applies now won't matter. They said in the battle reform blueprint that they wanted to make sure battles were challenging and you could not win with just auto attack.

Edit to add:

Can you even give me one instance in the game where you actually need to stop attacking the mob for a brief period of time and then during that time you can build you can build your stamina back up?

killing wolves its best to wait for full stamina and burst them down so they cant steal hp, anytime you run from an enemy's ws you lower your dps in an AA/stamina less system, but in a stamina system your stamina is recovering and you can go back and burst again. If you put a monster to sleep you can wait for stamina and burst it. When you do a battle regimen you are best off using an amount of stamina proportional to how long you think people will take to put their skills up before you go.

Most people didnt learn stamina because parties are easy mode within 10 levels, if you fight low man, or did big groups before and actually needed to adapt to your enemies, stamina was a very useful tool so you didnt lose dps for not doing something in one small instance. people never learned to use it, or learned the tricks of various monsters because the new sp system makes hard fights unworth it.

Stamina was very hard on tanks, but they could have altered that.

stamina was used to balance the cost of skills making them worthwhile, a big reason on evade/dodge/parry skills were worthwhile was because they cost little stamina. Its not the only way, but it had advantages

Be sure they will use cooldowns to manage skills, they have already said they will. the questions are how much, and what other resources they will use to balance the skills. If they thought something needed half your stamina bar, they will make a system that mimics that cost in some way shape or form.