It irks me that this boils down to cost. For a user like me it's about UTILITY and FLEXIBILITY. I want a compact system that fits in a single case and can fill several rolls with a quick parts swap. Like on my AR15, I have a nicely tricked out Ruger 1022 that a drop in 22lr AR kit can't touch in accuracy or reliability but that doesn't diminish my interest in a CMMG top of the line 22lr conversion kit.
Having a single Pelican case with a BCM carbine with both 5.56 and 300BLK uppers with a 22lr kit means that single case can get tossed in the back of the Jeep and fill multiple roles with less to keep track of, less space and overall lighter load to accomplish the same tasks as 3 different rifles.
I wouldn't care if a SCAR caliber conversion was $1500, I'd buy every caliber that I use and not blink an eyelash about the price.