That’s interesting, it’s exactly the opposite. The MS-MX packs are designed to support swapping.
They tried to make an affordable option available for the Roadster but the pack technology was so ancient that the best that they could do was to hand build packs. The MS-MX can easily take advantage of the latest cells and it’s extremely easy to replace the packs.
When Tesla has advanced cells you can pretty much count on them providing MS-MX pack upgrades at a reasonable price.[/B]
You can swap the pack, but only to one that fits the same dimensions. Fitting 2170 cells in the space for a 18650 pack probably means arranging them differently from how they're arranged in every other application, which means expensive new tooling, designs, etc.
And if a "X mark two" or whatever comes along that has newer tech with much greater range, it's pack isn't going to just fit the old X, they'll take the opportunity to redesign nearly everything on the vehicle based on what they've learned since.
That's not to say they couldn't build a pack that fit the 18650 dimensions but used 2170 cells, just that it would be expensive to do so (limited production, new design, etc). Just like the Roadster upgrades. Plus the cost of a whole new pack isn't exactly cheap to begin with. Being generous, let's assume 150kwh at $100/kwh price (could be a thing in a few years) - that's $15k, and that's assuming mass produced parts. Throw in the monkey wrench of limited production runs or hand building because this frankenstein pack isn't going to be used in 50,000 cars, and the price goes way up.
Of course, they might opt to build new 18650 packs using improved chemistry, but a good portion of the 2170 improvements are just from being larger cells, and thus less "wasted" (packaging) mass per unit of energy. I don't think they'll do that, because the market for these pack upgrades even at break even ($100/kwh) is just not going to be that big. They'll probably just use the new hotness to leverage people into buying new cars, then resell the old ones on the used market.
TL;DR: It's not that they can't do MS/MX upgrades, but I don't think they will, because the number of owners willing to shell out for a new battery will cause the battery to be way more expensive than you might assume from mass production $/kWh costs. And because the low volume increases costs, that decreases volume further, ... until they're in balance. And I just don't think it's worth anyone's time or money at that point.