Just because a game is out of print doesn't mean it isn't still an active game which is why going by 2015 and later is inefficient and would give you false numbers. There are a load of players still playing d6 Star Wars due to simply being a good game with a ton of material available for free and no proprietary dice. They also reprinted the anniversary book a year or so back.

Hell, AD&D 2nd and 1st editions still have a very popular following with large numbers of people playing them and they are what 20+ years OOP? I would venture to say they are likely the 3rd most popular ttrpg being played even.

I personally don't know anyone playing Robotech or Rifts but both are difficult mechanics games to play and the latter is still in print and releasing new material. I do know people still playing the long out of print Heroes Unlimited from Palladium. I am aware that the Savage Worlds version of Rifts seems to be popular so there is that and it is likely a strong #3 of popular sci-fi games.

Just at a guess I would say Star Wars in general split between the three systems is easily #1 so pick your poison on which. Starfinder is likely #2 because it is in print and is being pushed by a strong and popular company. #3 would be Savage World Rifts mainly because I hear a lot of chatter all the time so that might just be my little bubble. I honestly have never seen or heard anyone talk about playing or having played Traveler. I know of the system but not much about it.

These are just sci-fi. If you want to class cyberpunk as sci-fi then Shadowrun and Cyberpunk are probably neck and neck for #2 and #3 easily pushing aside everything except Star Wars. I think if they keep pushing Cyberpunk Red then it and its so much simpler system will eclipse the dice nightmare that is modern Shadowrun in the near future.