Street Fighter 6 achieved 5M units sold in 9 quarters (the 5M data is from March 31), while SFV took almost twice this time, 19 months, to achieve that:
SF6 achieved the 5M units sold as of March 31, 2025, months before releasing in Switch 2.
3rd party games, and particularly (real) fighting games sold considerably worse in modern Nintendo platforms than in PS or PC. So pretty likely will be the case for Switch 2. This is one of the reasons of why many fighting games weren't released in Switch.
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Tekken 8 is performing worse than SF6. Notice the difference in Steam's active userbase:
Previous entries as reference: