TK multiball is in the Pro already, and this idea would just kill Premium sales, everyone would just buy the pro and upgrade.
The idea or the tiers is that if you buy the cheaper model, you don’t get all the features, sell the features separate and you also piss off the high tier buyers.
You missed the boat on the premium, so you need to pay what the secondary market is priced at, pretty simple.

but it devalues the thing people already bought, thinking they wouldn't be able to have those features without buying the higher end game.
Also, they could make more in the future.

Ha, that's all fine. People have been trying to upgrade low tier games into "LE's" for some time now, often with great results, often not-so great.
But if Stern starts selling everything but the little plaque and Certificate (that aren't even on Prem.), are they really still pro's?
Some people bought a JP topper just to have the exclusive GOAT mode, and now they add it if you have Insider Connect+. That could hurt sales of a future topper with a mode included.
I have all tiers of machines, and I actually don't care if someone wants to try to dress up their game and make it blow bubbles or whatever, but i can totally see why STERN doesn't want to sell tier upgrades seperately.

...if you buy Prem./LE, why would you need a kit?