Good point, but I think the main issue is SSD prices are crazily competitive. In the same day I read XBS is getting a new 1 TB upgrade
for £150, I read a 5 GB/s nVME 1TB stick is available
for £44*. Unless there's rapid competition and downward pricing for XB peripherals, or plenty of notable discounts, it adds a large premium to the reduced-storage entry-level boxes.
Still, a first step implies future products from other companies. And even if not massivley cheaper, margins are probably fatter and attractive to more players.
* Yeah, it's a deal, but deals on big stick SSDs are way more common than for XBS storage cards as more producers and buying options.