I feel like smiting only on crits is emergent optimal gameplay: not metagaming, but definitely powergaming. I imagine the 5e creators wanted people to be able to choose when to smite so that players didn't, for example, waste a smite on a low-hp enemy that the normal attack would already kill. It just so happens that the wording + limited spell slots + crit mechanics encourages smiting only on crits.
Or more realistically, at least in the games I've played, paladins still do smite on some normal attacks. It's just that they ALWAYS smite on crits, and those events are so much more memorable because of the high damage that it gives the impression of smiting only on crits.
And as always, -1 for a hotbar "smite" icon, unless it's a toggle that allows other abilities to be used simultaneously and/or can be left on for use with opportunity attacks. Or +1 to pop-ups, but again there should be a toggle "don't ask me about smiting when toggled off" to prevent continuous pop-ups.
