remember that most people on this sub would, and most people on this sub probably know how to install RAM/graphics/IO cards onto thier PC, and even  if they haven't would be perfectly able to.

the general public... not so much. in fact sony's solution is likely to induce confusion.

Its probably part of the same reason the switch does NOT support bluetooth headphones, as it can't connect to 8 joycons and do audio at the same time, even if the other technical issues with bluetooth didn't prevent it.

I definitely like MS method SSD cards, but I also have zero interest in spending $300 on an SSD so I accept that Sony solution was kind of a forced hand because of the speed of their SSD.

I honestly will probably not expand storage for either system and just have a 2TF HDD to back games onto

I still don't get why it costs so much to begin with. Like the SSD speeds are less than 970 Evos and last gen WD Blacks. If they were like, well 1 TB costs $80 less than a new 1TB Samsung 980 Evo that's needed on the PS5, then that would be a pricing advantage that they can market.

The interphase (the connection slot design) and the way the SSD needs to be buildt around it..... makes it unlikely other SSD makers will want to inter the market. Seagate will probably take like large chunk of the pie so, it depends on how well the Xbox series X/S sells. Its probably a expensive gamble for SSD makers to start designing these.

Also the "only" downside? thats a rather huge one. Its gonna be the more expensive way to get a new SSD in.
The plug and play design is neat though, and I like the idea of swapping them in and out, if you happend to buy alot of them. However at 200$+ at peice, thats unlikely to happend imo. 

Research & developement.
The slot design with hot plug and play (pull in and out) and the size, determins the shape you need to design the pcb, and place the memory in ect.
Not to mention heat issues with such a small compact size.

Its basically a new SSD line (like they do for pc), for ever developer that chooses to make one of these Xbox series X ssd's.
Thats the downside to proprietary memory/storage solutions, costs go up. Mass scale is what drives prices down, and that isnt happening with something thats propriatary and not likely to sell more than 30-40m or so units (of SSDs) (not every xbox user will buy one of these SSDs for their console).

Seems fair to recall the outcome of Sony's own proprietary memory format, Memory Stick.
That failed despite being supported across Sony's entire consumer electronics line up.
Sony did licence it to other companies, but as limited market it ultimately never worked out.

Seagate propriety tax.

Or they might be using 2-bit cells on the NAND for durability reasons rather than 3-bit or 4-bit.

Once other manufacturers jump onboard the price should come down, but it's never going to be as cheap as PC SSD's due to the propriety form factor.

Nah. I don't agree. The general public can build IKEA furniture. They can slide open a panel and put in a stick with a screw.

able to and willing to are two different things, I know quite a few people who just buy gaming PCs from people who put the machines together and never open it up, despite the fact that this method costs them 70% more then buying a decent desktop with decent CPU/motherbord and putting everything in

That's very different. If people aren't willing to upgrade their SSD's then they wont' have to. These are the same people that weren't willing to upgrade their internal storage last gen, they still bought consoles. This method of upgrading storage wasn't a dealbreaker to the 113m+ people that bought a PS4.