I think a new Halloween stage means that Capcom isn't going to be done with SFV for another couple of years and that they want people to still think the game is still constantly being refreshed. IMO, if support were ending next year they wouldn't bother releasing more holiday stages.
I guess I'm willing to accept the belief that SF6 won't hit until 2022. That means SFV's lifespan will be exactly the same length as SFIV's (I thought it would be a year shorter) .
Maybe 2020 will bring in another 6 characters. I doubt there would be 6 characters for SFV in 2021 though. Capcom will probably be promoting SF6's roster by Summer 2021.
I wouldn't necessarily expect a 6 character season next year or in 2021, but they're certainly gonna keep drip feeding content until 2022. They gotta keep the game active until SFVI for CPT. Capcom will not let it lapse with Bamco, NRS, SNK, and ASW trying to make their pro tours out grow the CPT.
We're due for a title update in December or January. Street Fighter historically gets one every two years. If we get that, expect a rerelease at the start of next gen: a complete edition with all the content that's on par with PC at max settings.
I'm pretty confident we'll get at least that because this was the exact roadmap that took us from USFIV to SFV.
My hope is we get to an even 50 characters. I'm not so confident about that. That would require two characters this season (all but guaranteed considering the leaks), one more "season" (not unreasonable to expect, but not certain), and four more characters after that (a long shot).