Well the survivability was with one ship against multiple Borg Cubes which is better than Starfleet ships would do in that situation seeing as one cube can take on a whole fleet.
I don't understand why this is even an argument. Yes, obviously the entire point of bringing back armor from the future is that it's a better defense than contemporary shielding. But "better" does not mean "absolutely impossible to defeat." It just means it gives them an advantage they didn't have before -- but, as is always the case with the Borg
, that advantage only lasted until the Borg managed to adapt, because that is what they do.
"absolutely impossible to defeat."? No.
Being able to take 100s of times more punishment than conventional shielding without failing? Yes.
Realistically speaking, no one (except, perhaps, suicidal luddites) would throw away such a technology due to a thermodynamic issue such as the one you technobabbled - one that, as Endgame showed, didn't prevent the shielding to function well for an extended period.
And the borg's ability to adapt to anything is just as magical - if not more so - than the armor from Endgame.
Yet you seem to have nothing against it (much like warp, transporters, etc). Whatever your criterion for choosing what's 'realistic', it's not scientifical plausibility.