I disagree, for starters, the game not being multiplat is the biggest reason it's not as popular. Street fighter 3 was never popular, only internet folk hero's have played it more than thrice

SF4 was considered the return to street fighter 2 style gameplay and that's why the masses fukked with it. From what I have gathered, sf5 problem is that it wasn't ready. When it finally does get there will people still care?
in the meantime, street fighter 4 is backwards compatible on Xbox one right now
In regards to 3rd Strike, it was a game that had a very big skill gap compared to 4 and it's mechanics. The introduction of Ultra's wasn't accepted at first, due to the fact it's a comeback mechanic and rewarded players a strong attack for essentially taking a beating. There was a skill in using it/comboing into it but comeback mechanics are always going to be met with criticism. There was balancing issues with characters and complaints about the game being 3D. Vortex style became pretty non-existent and the Focus Attack got a lot of flack in the beginning for impacting footsies, but eventually players found its place within the game. 4 was a good attempt at returning more to the style of 2, but there were things at work that ended up ultimately making it its own game more than anything. People enjoyed 4 overall at the end of it's life but there were still complaints about 4. I think that same pattern will happen with 5.
As far as popularity, it's still doing alright. As far as only being on PS4/PC, sure it does hurt it's potential fanbase, but a great way to make-up for that is the ability to play across platforms. I can plug my fightstick up to my PC or PS4(I own a copy on both platforms) and start up a lobby and it can have PS4 and PC players in it. Something that SF4 never had. You had to have the same gaming system(360, PS3, or PC), and then you had to make sure you had the same iteration, and each iteration was usually another purchase entirely. With 5, I don't have to worry about those things because all the updates so far have been free. I don't have to buy new characters to play against new characters, and I have access to 2 install bases worth of gamers to fight against with cross-platform compatibility instead of one like when I played 4 on XBL. And as far as tournament play goes, SF V still has a healthy flow of entrants that matches and/or beats SF4's entrant numbers. Part of Street Fighter popularity is the e-sports aspect to it now, and if anything it's being presented in the best way possible with TV spots and player sponsors. It's always dope seeing people randomly tweet out that they're watching Street Fighter on ESPN even though they aren't even gamers cause that shows it can reach a different world of people. I think the popularity for SF V is going to be gauged a bit differently now with the way technology can help bring new people into the community on a local, national and global level.
SF V has it's problems for sure, I'm not denying that, but it's still got a healthy player base and when it's finally in a 100% polished state I think people will still care, simply because they do now, after a year and some change of being released. We're seeing Capcom listen to feedback and change things and while they did drop the ball on some major things, the core of SFV is still pretty in tact IMO. The Capcom Pro Tour is a great idea, and them taking from Dota's book and releasing special aesthetic content for a price, and putting those proceeds towards the prize pool is always going to keep your game afloat. New games always drop players but they also bring in new ones to balance it out. I believe SF V is currently alright and will continue to be. SF has never been a casual series so I think people that are still sticking around will continue to.