They are not a guaranteed return because you can't be certain they will in fact make the difference between "teammate on the ground" and "teammate up and fighting" for at least 1 round. That is the virtue of the "whack-a-mole" strategy: If there are no enemies between you and your teammate in the initiative order, you know with 100% certainty that your healing spell will purchase at least 1 round of actions for your teammate. And if there are enemies between you, you know that, and you don't cast the spell in the first place.Why aren't my proposed healing tactics not a way to get a guaranteed return?
However, an investment does not need guaranteed returns to be a good investment. It just requires a risk premium: The greater the uncertainty of the payoff, the bigger that payoff must be to justify the risk. Mass cure wounds cast by a Life cleric is a great example: If you do it when the entire party has taken some heavy hits, you have a good chance of purchasing 2-3 rounds' worth of actions (one for each teammate who is saved from eating dirt for a round). Or suppose the party tank has a stratospheric AC, such that they rarely get hit. In that case, a big healing spell could purchase 2-3 rounds for that one character. The results are not as certain as whack-a-mole, but the potential upside is much greater.
And if the DM is playing monsters "viciously," so that they go hard after downed PCs, the potential upside is an entire adventuring day's worth of actions (or however long it would take a slain PC to be resurrected or replaced). However, this is a case where you are spending an action this combat to buy actions in future combats, so you would only do it if you were fairly confident of winning the current fight - if you're on the ropes and facing TPK, then future combats are irrelevant, the focus must be on surviving this one.
I more or less agree with you: In-combat healing can be a useful tactic. It's just a matter of figuring out how to get the best value from it and knowing what spell to use when. That depends on your party composition, the adventure, and the DM.