they'll never do that, as long as they can build and sell you a complete new premium unit.

But that's just it, Premiums are no longer available from Stern.

I would love a Stranger Things Premium, but I'm not going to pay the jacked-up prices people are asking for them.

If I could buy a new Stranger Things Pro and an "upgrade kit" to add the missing features of the Premium / LE, I'd buy them both in a heart beat.

Upgrade kits would be a huge seller. I'd pay up for the kit, many would. Name your price stern.

I'd be down for a projector kit!

Why wouldn’t you be able to buy the projector as a part, install the premium code and disable the telekinesis lock? Are there any additional boards for the projector?

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 Stern isn’t making Premiums anymore. From their perspective, there can’t be any cannibalism of sales.

Disagree with DNO completely.
I am a high tier buyer and think it's perfectly fine if others upgrade their machines with the cool mechs. They're still pros at the end of the day.
Folks will still buy the premiums and LEs at high prices because people in this hobby have money and like to show off their high-end machines, not a thing wrong with that.
I think everyone should have the opportunity to add these mechs, really cool.

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?

Once the game is "retired" or "discontinued" as far as Stern's OEM sales are concerned, why couldn't they go back and offer what is essentially an upgrade kit at somewhere near or even above the MSRP difference of Pro to Premium?