Is there a way to have more than 15 Program Change slots?
Why the limitation?

I absolutely agree. More Program slots would be greatly appreciated. The more the better :-)
At the moment I have to load 4 instances of Microtonic into Ableton which is not a very elegant solution.

Unfortunately the same with Synplant, very few Program Change to chose from as well.

So you create an instrument rack with several instances of Microtonic to have more program changes? That's a good idea. Although cumbersome I agree but hopefully this can be addressed in the future :) Those instruments are so cpu light that it makes them ideal from program change trickery. I use Dial to Program Change, it's very cool.

Dial to Program Change - haven't heard of it before. Sounds interesting. But unfortunately I'm not too keen on Max For Live devices especially for a Live set being used on stage. In my experience they crash quite often.
As you said: I'm using an instrument rack where all the instances that aren't in use are deactivated. So resource wise it shouldn't be a problem. But as I said: not very elegant. It feels more like a cheap compromise.
I already begged for more program change slots a few years ago. Unfortunately to no avail....