Artillerist Artificers are amazing at it, honestly.I mean, whether they're about to drop or not it actually can be helpful. It's just if they're not you're guessing at the future. If someone seems likely to take more damage, then yeah shielding them is likely to be better. If they're about to drop you can be pretty much sure it's better - so that just removes the guesswork.
As for keeping PCs "at least one crit away from the ground", I don't think that's even physically possible until like, what level 5 or 6 or something, given how big some low-level monsters can crit. Even then it largely means keeping people topped up until like what level 8? Obviously some PCs have waaaaaaaaaaay more HP so will meet this threshold sooner.
Drop down a protection cannon and use your bonus action to make it pulse every turn for 1d8+int Temp HP every turn for an hour. Or keep it on your person, however you prefer to handle it.
You can keep the whole party bathed in a lovely glow of Temp HP!
So, y'know. It's gonna depend on the class, really. Some are great at it, some are okay at it. Some of them are better at last minute saves, like the Psionic Knight.