Doesn't really work that way though. First of all you're assuming I guessed exactly which character would take the 100+ damage, but you don't. In a typical party you need to have 125 THP granting to equal that 25 HP healing. Certainly you need a lot more than 25, or you need to be assured that everyone in the party is taking a pounding, and even then most likely if damage is spread out that much then healing would serve equally well or better.
There's also the question of just exactly how durable THP really are. Actual hit points are permanent, and this is true in all situations. At least in the game I play in a 'long rest' isn't something we just get automatically, our characters just finished trekking for a week in the barrens without getting one, THP would be long gone in that situation, healing would remain.
The real problem is you can't reliably state what the equivalence factor is, it is highly dependent on the table, which rules are being used, etc. I'm pretty sure that's why 4e used mechanically similar healing for most leader classes.
TempHP are just as permanent, nay more permanent than regular hit points. Sure HP restores over a long rest - so does whatever was used to grant the TempHP. Sure TempHP is gone after you take damage - so is regular HP.
TempHP, however, is completely unaffected by your Hit Point Maximum. It remains even if you Hut Point Maximum is reduced. It can be applied even if your at your Hit Point Maximum. The only thing that it can't do better than HP is get you back up from 0HP.
Your example's also completely wrong. Let's take the classic Inspiring Leader. It lasts until it is depleted and can be reused on a short or long rest. Only way a healer can put up that level of output on the described trek is if they're a Warlock.
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