What's the difference between this watch and a base model non-Pro Epix gen2?

The name.  That's it.

And the price of course

We need to talk about the Garmin Fenix E

Chase the Summit

https://youtu.be/oMficmJn4BQ 

Is there a new cpu and/or gpu chip that's needed (under the hood in the Fenix E) to run this new interface? In not then WHY would you not bring it to as many devices as possible? Two reasons, it keeps customers happy and away from products that use literally the same interface across multiple ranges 3-4 years of devices, and it also saves your software team from having to manage yet another code base for a couple years.

Because it funds both current and future product development?!

What industry do you work in where current live product development can be sustained by old revenue that is already off the balance sheet?? 

Likely software.

Software Engineering! What industry do you work in that doesn't have competition ready to eat your lunch? You can't just keep spitting out slightly updated hardware, or hardware not updated at all, and not backport software in this day and age. Competition will eat your lunch! Even the most closed ecosystems do this

They said that the base code is completely different and not compatible with older models, whatever that means for a common person

Most likely it means that the code can't just be copy/pasted from the new watch to the old watch and still work, it would require some degree of adaptation to work properly on the older hardware that, frankly, just isn't worth their time to do. And honestly that's fine by me, I don't particularly really want the new UI on my Epix, I like it the way it is.