This may be a table thing. 1st spells still give +2d8 damage, +3d8 at 11th. Double that if applied on a crit. And a 1st level spell is not a hefty opportunity cost.
Even for a higher level spell doing 18 HPs of damage - preventing a damaged foe from getting another turn can easily be worth 70 HP of healing after a combat ends. Easy example, dropping a caster who would throw an AoE spell at your party. Again, that's not saying always, but it is saying a common conditional. Probably comes up at least once per combat where an appropriately sized smite would drop an opponent but a normal hit will not.
When I first read the title for some reason I saw Night Smurf!
That got my attention!
Sorry got nothing actually constructive to add.
It costs you your opportunity to do 2d6 damage with Thunderous Smite AND possibly knock the enemy away and prone, or 1d6 psychic damage with Wrathful Smite and a good chance to render it combat-ineffective. Is the extra 2 points of damage (Thunderous Smite) or 5.5 points of damage (Wrathful Smite) really that critical to your combat strategy?
RE: "smite on a crit." It's not really compatible with the initiative system I use (everybody declares, then everybody acts), and my players find smiting on a crit absurd anyway (at least, I remember one paladin's player openly mocking the idea of deciding AFTER the hit lands whether or not you're going to smite), so I have no experience actually playing with it.
It isn't enough for an appropriately-sized smite to drop an opponent where a normal hit would not. You have to do that on a turn where the enemy is about to inflict 70+ HP of damage on the PCs before anyone else can act. Could be a table thing, but IME that's quite rare, outside the glassy glass cannon Meteor Swarm scenario already mentioned, or equivalent ones such as an already-damaged Flameskull casting Fireball when four or more PCs are in Fireball Formation.
Since players (at my table) don't know the monster's remaining HP or spell loadouts, they wouldn't even know for sure they were in this optimal scenario even if it did arrive--you could waste your spell slots on smiting AND STILL get Fireballed, and now you will have trouble even healing the damage because you already blew the spell slots.
Divine Smite isn't a bad capability to have, but I predict the proposed nerf (basically removing it from the game entirely) will have little impact on game balance.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.