"It has always bothered me that we might have to accept that the Borg were always supposed to look like how they did in First Contact with no explanation at all except because TNG didn't have a big budget."
No offense to you personally, but it's always bothered me when they try to "explain" the obvious, making farce of it all. Wouldn't we be better served by having the writers create new stories, ever better ones, or have the effects people retouch things than come up with lame fourth-wall shattering excuses for the nature of real-life reality? Make-up gets better, effects get better, cameras get better, language patterns change, social norms change. I don't care as much why Klingons look different between TOS and TNG as much as why they even matter any more. When we saw them last, I had a hard time believing they could even pronounce "phaser," let alone build one.