Doable, though a little more intensive on data tracking than my personal taste. i recognize the fundamental weakness of that objection, especially in light of guardian of faith. (But it’s also why I don’t like guardian of faith.)

I agree, I don’t think it’s a great spell, but it has its moments and for a Warlock most other attack cantrips are pointless given the way they made Eldritch Blast work (bleh) so it’s not totally off. It’s not too bad in the mid-levels, too, as there you start running into foes with resistances and can’t really count on everybody having a relevant magic weapon. Recall there are cases where the melee combatant might need a missile weapon, and Magic Stone provides that.

It would be a much better spell if it were leveled and could be prepared, say generating some amount of magical ammunition, though, or if it scaled better by level. I think WotC often thinks that something scales by virtue of level advancement alone and were frequently wrong about how relevant or powerful something would be.

They also had some notions of trying to make a game that had always been driven by magic items to no small part (in some editions too driven) to have no magic items. So I think some spells would be much nicer in a non-magic game.

On magic stone – sure, but that means the spell is at its most useful when you a) have the fewest cantrip choices and b) are, overall, least likely to face opponents with resistance to nonmagical B/P/S damage. Yes, you can play a melee combatant up to high levels without a magic weapon, but I suspect that if that keeps happening and the fighter keeps getting relegated to using the druid’s magic stones, the player’s going to get frustrated in a hurry.

IMO one thing WotC tends to be kind of bad at is making things have lasting value. Magic Stone and Familiars are both good examples. At low levels both are much more useful than at medium to higher levels.

Your point about Crown of Stars vs Melf’s is also telling. WotC seems to slap Concentration onto spells in very arbitrary ways (boo hiss… I hates it, Bagginses). Having a class of spells that last a while, activate with a bonus action, and don’t require Concentration works out nicely to allow for some flexibility. The action economy provides a useful limit to how beneficial this can be and if you’re walking around with a ton of precast spells, you’re one Dispel Magic away from having those get dropped.