They've done it before
Intel tried this before, in 2010, see here: https://en.wikipedia.org/wiki/Intel_Upgrade_Service
This was ... controversial. I'm going to explain what Intel did, why they did it, and why people don't like it. Please note that the downvote arrow is for if I've done a bad job explaining it, please don't downvote just because you don't like what Intel are doing!
Retail shops that sell complete systems can't offer the range of CPUs that you can get online. So Intel cut a deal with certain PC manufacturers, they made a special CPU and artificially disabled some bits of it. When you buy the PC, the shop can try to upsell you a card with a code that you can use to enable them. This way, you can choose to buy the cheaper CPU with less cache, or the faster more expensive CPU with more cache and hyperthreading. And the store doesn't have to keep two different models of expensive PCs in stock. This is good for the store, they can potentially make more profit.
Note that Intel "bin" their CPUs. This means they make a lot of CPUs, test and sort them based on the working features and the speed they work at, then label each chip with the best part number that that particular chip is capable of. "Best" meaning most profit for Intel, obviously. If they are making too many expensive chips and not enough cheap chips, they would normally permanently disable some working parts of the chip, so they can label it with a "worse" part number. Presumably in the case of Intel Upgrade Service, they did that but disabled the working parts of the chip in a special way so it could be re-enabled with a code. They could then sell that for the price of the "worse" chip, but potentially make more money later when they sold the "upgrade".
However, many people didn't like this. They felt that if they'd bought a chip which had certain features inside it, they should be able to use those features without having to pay more to enable them. It brings up a whole host of policy issues. It's pretty much "DRM for hardware", with all the politics around that.
Intel's attempt in 2010 was limited to one processor, and was extended to another three in 2011, but it was discontinued later in 2011. I guess Intel want to try again.