Whilst I'm not a massive fan of reboots and remakes, they don't always have to end badly. The remake of Robocop turned out surprisingly good, if not amazing. As did IT.
However, Back to the Future is one of those unique properties where lightning really only struck once. Much of it's likeability is down to the cast and their dynamic, but the combination of everything - the score, the setting, the script, etc - just worked perfectly. I just can't see them replicating that in any way that's better than the original, and if they can't better the original, what's the point?
I'd possibly be open to another sequel but only with as many of the original crew and cast behind it as possible. And given they're all getting on a bit, that's increasingly unlikely.