If they had marketed this as 6e, with enough serious changes to actually justify it buying it, then even if it were still backwards compatible with 5e, I probably would have been far more interested in getting it.
Imagine the uproar if they had called it 6e but then claimed backwards compatibility!
But I get what you mean - I'd have loved to see a totally different take on each of the classes, but designed to play alongside the 2014 ones interchangeably. I don't really care if you can mix-and-match elements
between them (in that I'd be happy if that answer was "No") but it would be good for them to keep their primary goal of being able to run them at the same table and the same adventures as 2014.
Even more than PCs - I'd like to see a
completely different take on Monsters, mechanically. If they kept the same (well, actually
better would be good, but roughly the same) relative balance when it comes to Monster NAME to Monster THREAT, then the adventures would still work, too, even if the monsters "ran" differently.
But their current plan is much SAFER, which might lose people like you, who are more interested in experimentation for a chance of improvement, but it'll probably bring in higher numbers their way.