I disagree.
First, there's already plenty of at-will support in the game.
Help action, grapple, viscous mockery, mantle of command, paladin's aura, guidance, warlocks repelling blast, light, minor illusion, resistance, spare the dying, blessings of the trickster, and any level 1 or 2 wizards spell. Not to mention, things become effectively at-will. Mid level clerics don't avoid casting bless because of spell slot limitations (concentration is an at-will resource).
Second, do you really think the at-will help action that everyone is breaking the game? Does the game break down because wizards get at-will feather fall? Because I rarely see anyone use them. Even a mastermind rogue will often forgo helping to TWF (when he misses his first attack).
If your option was to use your action to grant everyone +5 AC and saves against a dragons attack, or to kill it, which one will save you more HP?
I XP'd this not because I agree with it, but because those are all good points, and I have no valid counter-points.
I'm not sanguine about it, though. It seems like resource-management and daily (and other recharge rates) resources are too central to D&D in general, and support dynamics, specifically, to be just cast aside like that. But it's just a feeling.
The only thing you don't want at-will is healing.
Ie: Regeneration, Useable on Others. Agreed.
It's in the nature of RPG mechanics that the player gets to affect the fiction. At it's in the nature of reaction/interrupt-type effects that the player gets to contradict or deny something the GM was putting out there.
Sure, absolutely. And, it's the intended nature of 5e to be very DM-Empowering. So, while it's fine for PCs to have abilities and backstories and spells and take actions and all, all of which affect the fiction, whether they can contribute to the DM's domain - the plot, the actions of NPCs, the facts of the world outside their immediate/direct influence - is something the DM should have to allow when it's OK, rather than revoke when it's not. Players need to be set up to be grateful, rather than disappointed, so as not to tarnish their trust in the DM.
(I know, illusionism, force, agency - I get it, you're not wrong. But I'm OK with those things, and, I think, it's the bed 5e has made for itself - knowingly, even, and in which it is reclining comfortably.)
Rather like skill checks, really. It's not like something has to be major, like returning the dead to life, for the DM to have (need) ultimate control over it in 5e. Something as mundane as opening a door is the DM's to declare successful, require a roll, or judge impossible.
Spells are push-buttons or coupons or however you want to think of it, that simply do what they say on the tin, and the DM vetting every spell like he does every skill check & action probably wouldn't go over well, because there's an expectation of privilege when a spell is cast. There's no such expectation when trying to shout the probably-mostly-dead back to probably-somewhat-alive, so that formal DM-may-I process could probably be unobtrusively inserted in front of such abilities to head off any such 're-writing the story' issues before they ever came up. Like "..it may be that all who seem dead are not truly dead, at least not yet, and that the devoted, the determined, the heroic may yet miraculously save them..." even if only flavor text, would imply that the ability is something that may be usable, when the situation warrants - and the arbiter of the situation is always: the DM.
