Or, and hear me out, it calls for teamwork, tactics, and a dash of gritty realism. Though I doubt this is what WotC had in mind.
It's definitely not what WotC had in mind, and further, it doesn't work well at all.
A group who heals heavily in combat will perform far, far worse than one that focuses on killing the enemy.
That's not even debatable. That's simple math. WotC decided to make the healing largely combat-ineffective and a waste of an action.
And there are the amplified problems of "what is a hit point," " how is damage reflected by my character, " and the new one: "why does my spell behave differently when a goblin looks at me menacingly?"
None of these problems are new, as you admit with "amplified" and none of them are significantly worse with this. Even the last isn't really new - lots of stuff in 5E functions in way that are practically different in combat, and trying to phrase it poorly to make it look bad just shows a lack of a rational or logical argument.
My plan totally fails to recognise "wasteful" healing for a reason: it's not wasteful at all. It prevents yo-yoing. But hey, if you like getting your lights knocked out and continuously wondering where someone kicked your weapon this time, have at it.
My entire point is that wasteful healing
does not prevent yoyoing. That's why it's so wasteful lol!
When you can only heal a single character at touch range for 3d8+4 (17.5 avg) with a 3rd level spell - the equivalent to Fireball, Call Lightning, Counterspell, Slow, Fly, or Spirit Guardians, yet the monsters you're encountering frequently hit for hit for 2d8+4 with a single attack, and have multiple attacks, you're spending your whole Action, your movement (if it's even possible without provoking OAs!) and a 3rd-level spell slot (!!!) to, at best, eat the damage done by about 1.5 enemy attacks - with some enemies at that level it's less damage healed than a SINGLE attack does!
Thus it doesn't really prevent someone being downed - it
might delay it by a single attack, maybe two on a very good day, but you're blowing a third-level spell to delay downing by probably part of a round. A spell slot you could have used wildly better.
Being downed eats a ton of extra damage - by WotC's intentional design. You trying to make it more punishing without actually even thinking about or looking at the action or spell slot economy involved isn't good design and it's not good DMing, frankly. It's classic knee-jerk DMing that doesn't look at problems holistically, but misunderstands them as a series of separate and unrelated parts, not a whole systemic issue.
I get that you don't like the THP, but it actually largely solves this problem by making heals twice as effective in combat. It's not a great use of a spell slot in a lot of cases, but it's drastically closer to one when you can basically guarantee to be equivalent 2-4 attacks instead of 1-2. It's not tactically optimal but we don't need it to be - just to be reasonable - which combat healing currently is not.