Well, for starters SF6 sales on Xbox would in fact inform whether it's worth porting the Marvel fighting games, because Capcom can assume that the Marvel collection will sell some fraction of what SF6 did, and assume that ratio would be similar across platforms versus SF6 sales on those same platforms.
We don't have SF6 sales splits for Xbox (yet), but they'll probably leak in the future. Going by other leaks, I would just say to brace yourselves for rough impact. The game probably did not move mountains on Xbox, which would make sense because it's a global franchise and Xbox is not a global console brand.
The "good Xbox sales" for SF were with Street Fighter 4, and apparently that didn't mean much to Microsoft because when they had the choice to help fund Capcom games at the start of XBO, they chose Dead Rising 3 over Street Fighter 5. Sony cared more about the IP, so they chose SF5. Exclusivity was decided on by both SIE and Capcom, so it's funny how only SIE/Sony get the "blame" for SFV being a PS4 console exclusive