Class D Amps with Plugins...

I'd say we're already there.
Fender Rumble Studio 40 40W 1x10 Bass Combo Amplifier

 

Isn’t that basically what the powered Kemper does?

 

While not technically plugin, Powered Kemper Heads uses IR's that way. Line 6 Native and Helix to a power amp is fairly close to that idea. The thing is between IR's and Plugin's, which is better? IDK

Take a laptop with class D amp, and you got it basically now, but then companies like Line 6 may want higher premium for stuff like Native to be used directly.

 

So basically an open architecture system?
Yeah. I see all the manufacturers giving up their proprietary systems to do that real soon.
It certainly makes sense, and I applaud your comments. But the world is littered with great ideas that never caught on due to turf battles.

 

Take a Laptop, or a Mac Mini, or an iPad and have at it.
Check out IK Multimedia Tonex - it's getting quite a following.
TONEX iOS

 

I've always had much better results using plugins in post processing than in real time playing. But I’m sure it’s coming.

 

Well honestly it doesn't even have to be a "Laptop". I could probably piece together something that looked like a nice cohesive amp with a single board computer, DI, and Class D power amp board today Now foot-switching would be fun, but probably possible too. Though, you would need a way to control it. That could be done with a small android device or some other remote method.

 

Exponent ⁵⁰⁰ – Darkglass Electronics

 

Nope. More stuff to go wrong with an amp. Patch in a multi-effect pedal and be done.

 

Helix

 

The Bergantino B amp and Forte have the architecture to do this, but they do not allow end user to make adjustments.

 

Every manufacturer that has developed products like this have experienced significant challenges. The cost of supporting this type of product needs to be spread out over enough units to make sense.

Just keeping systems updated and bug free along with being compatible with ever-changing external control devices and third party software is a missive job, which is why support for older products like this is dropped when it no longer makes economic (or technical resources) sense.