I am not an expert for DnD 5E, but I know that for many CC spells the target has to make a saving throw every round to escape the effect, so the better your DC the higher the chance that it lasts long.
In DnD 2E the spell did not depend on stats of the caster, which means the chances of success got worse over time as powerful enemies have better saving throws.
You only had to make repeated saves for environment effects like clouds or entangle.
Prof. Dr. Dr. Mad S. Tist
World leading expert of artificial stupidity.
Because there are too many people who work on artificial intelligence already
