As noted above, the errata from April made it clear that you do lose the temp HP when the spell ends.

Quote from KristianCastsFireball >>

Considering the vast majority of animal forms are still not that great compared to other options at the level you'd have access to Polymorph, I think using it for utility is probably the worst use case.

So you are only interested in spells offensively, for battle? Then, this spell is clearly not the best option for you.

Quote from KristianCastsFireball >>

Using it for an offensive form in the event that you're maybe down to your last spell slot or the barbarian is out of rages or something and needs a boost might be another decent boost, but the temp HP here will likely be way more valuable that anything in most Beast stat blocks. The alternate offensive use, the spell's primary usage considering it's a Wisdom saving throw spell first, is to incapacitate an enemy into some pathetic form and ignore it, but since it retains personality and such, it still remains a potential threat (albeit a negligible one).

 If the beast you are polymorphing into lacks the offensive capabilities in a battle, then simply pick another spell. This one is not for you.

Quote from KristianCastsFireball >>

Using the spell to take on a form and drop concentration to keep the HP though is totally viable as the only other spells giving you temp HP at this level are False Life and Armor of Agathys, both of which are far lower than what this can do. The next one up is Power Word Fortify which can split the 120 across targets. I think it's perfectly balanced as just a giant meat shield (...)

Exactly my point. Other temp-HP-spells not enough for you? False life (which can be upcast bytheway) just gives a measly boost, way too low for it to be worth? Is the spell level of Power word fortify too high for you and not worth the spell slot? Lets abuse this 4th level spell and get 144 (or whatever) temp-HP, instead of using a 7th level spell for 120. Abusing this spell, getting more HP than a spell 3 levels higher, whose intended purpose is to actually provide temp-HP, is okay to you, since RAW failed to specifically state the blatantly obvious fact that the temp-HP only applies while polymorphed?

Quote from KristianCastsFireball >>

I don't think "intended use" is just the unga bunga tactic of get big and hit things. I think the spell is designed to give players tactical options in deploying it. Plus the spell states that it's effects shape change and then LATER very specifically call out the temp HP as separate, in a whole new paragraph. That's on purpose.

 So using polymorph for the beast's utility, or using it to "unga bunga tactic of get big and hit things" is not what you are looking for? Then why, oh god why, are you even looking at this spell? Those are obviously the only intentions of this spell. Nothing else. At all.

No. This is still not a temp-HP spell. You are playing this game wrongly, trying to "win" DnD. Stop reading everything RAW, that is so dumb. One should not need to get into needless discussions because powergamers like you scrutinize every single word, trying to abuse the rules.

You need to play more RAI. Much more RAI. You really think they made this spell to give characters temp-HP? This spell is clearly, obviously, hands down, no doubt, certainly, surely, about changing shape. The temp-HP is just how they game-mechanically solved the increasing toughness that comes with turning into a beast. Trying to extract the temp-HP from this spell, without being polymorphed, is so dumb that I can't even.

Stop abusing rules by going RAW.

Also, looks like they changed it lol. I win. RAI wins. 

Yes, you are correct that RAI was different than RAW and now errata reflects the true RAI. Temp HP only lasts as long as the polymorph form lasts.

My favorite use for Polymorph will still be to offensively reduce the capabilities of an enemy, or to keep my party member in the fight longer when they have low health and no one has any slots for healing or potions left.