I'm interested in how else you were thinking of adding one, if not by opening it up and inserting it manually?
Well, given that it's a console for your average Joe Public, I'd say that a solution like Microsoft's would be how most people would expect to expand the storage in their games console. Not having to go through a list of NVMe options to find one to buy, figure out what heatsinks will fit it when it's in place, manually remove the cover and then screw it into the console and put the console back together. The last few pages of this discussion has been crazily convoluted for those trying to work out what to get for their upgrade. It's a console that's meant to simplify the gaming experience for users, not give them the PC upgrade experience.
For all the flack that Microsoft received, they sure did choose the best option and it was ready for launch, despite the initial cost. It's also turned out to be similar in price to the PS5 options, but it's so much simpler to use, and you can easily use multiple expansion drives if you needed to. Eventually there will also be other compatible MS-licensed drives so prices will come down even more.
I get that the PS5 storage is double the speed of that of the Xbox storage, but we've no idea what Velocity can do yet and that's meant to be the big thing to work with the drive speed for the Xbox consoles this generation. And I'm fairly certain that raw MB per sec isn't going to translate into that much of a difference in loading times and seamless area loading during gameplay. I'm sure it'll be pretty similar across both consoles.
I feel that Sony have let their users down with their offering. It just seems to be a rather confusing solution for a console that's meant to make gaming life simple and accessible to all.