The video at the base of this thread has iFixIt take the 500GB SSD from one Mac Mini and swap it with the 250GB SSD from another, and both recognized and worked with the replacement.

It is a swappable part. Which means much more attainable servicing for flash failure or exhaustion, and possibly even upgrading storage in the future.

What stops someone from selling replacement cards?

not wanting extensive interactions with Apple's legal team. Charging $400 for $10 of storage means they have a lot of money to harass you with very well payed lawyers even if you are in the right.

There have been companies selling hardware upgrades for Macs that weren't designed to be upgraded for many decades now.