While you are correct that spells like the smite spells and the ranger's Ensnaring Strike like spells aren't implemented properly, and should be fixed if Larian wants them to be properly balanced, I believe you may have a misconception of how they work in 5e.
Unless I'm misunderstanding you, you're saying that searing smite should apply to all attacks during the spells duration, but that isn't the case. It only applies to the first melee weapon attack that hits while you have searing smite active.
So while Larian's implementation of searing smite(and similar spells) is a huge nerf, because the proc doesn't sustain through misses, the 5e version's doesn't sustain through hits either. That would be bonkers and make spells like hunters mark, hex, and divine favor laughable in comparison.
