Tennis will always be a popular sport, I don't think people have to worry about its future. There are some interesting players. It's still in the top-5 of the most watched sports in the world. But the future doesn't look quite bright with all these Saudi Arabia rumors. I have read some unsettling articles about what the future of this sport may look like in a few years from now. If some of the things I've read are true, a lot of things are going to change.
Now if I have to talk about players popularity, well... Like it or not, the Big-3 was the Golden Age of this sport. Djokovic, Nadal and Federer were all unique for different reasons, and they elevated this sport to new heights. A lot of people born in the 1980s or 1990s grew watching them. They have BY FAR the 3 worldwide biggest fandoms. It's no secret that MILLIONS of people stopped watching when Federer retired. Other MILLIONS are slowly quitting as Nadal's end is getting closer and closer. It will be the same when Djokovic leaves, the majority of his fans will leave with him.
You will get the impression that Sinner, Alcaraz (and other young players) have similar fandoms and that the sport is more popular than ever but don't get fooled by it. It will only be because Federer, Nadal and Djokovic fans stopped watching the sport and stopped posting on social medias. Only the biggest, truest fans of the sport will stick around and pick a side between the new players. Most of these remaining fans will choose between Sinner and Alcaraz by default IF this is about to become the new big rivalry. But as good as these guys become and no matter how many Grand Slams they win in the next few years, the hype will NEVER be the same. Fan wars between Sinner and Alcaraz fans will never come close to the wars between Federer, Djokovic and Nadal fans. (Sad to see that a huge part of the Sinner and Alcaraz fans today are in reality hidden Djokovic detractors who only want Sinner and Alcaraz to stop him, but that's another topic). Even if Sinner and Alcaraz (or others) play epic Grand Slam finals in the coming years, those matches still won't get the attention of AO2009, AO2012, AO2017, RG2013, RG2021, Wimbledon2008, Wimbledon2019, USO2010, USO2011 and some other Slams Big-3 matches. Anyone who grew watching Nadal/Djokovic/Federer between 2004 and 2019 can never get as passionate for Sinner Alcaraz and the other NextGen or NextNextGen players. Thankfully there will be new young fans in the future (people who never saw the Big-3 playing live) and these young people will grow to like other players but it'll still never be the same. Sorry but that's true.
Speaking for myself, I'll still watch tennis even after Djokovic and Nadal put the racket away for good. But my interest is going down. It just won't be the same. I used to wake up in the middle of the night to watch Grand Slams or Masters matches. When I was at school or high school I used to hide a phone under the table and secretely follow tennis matches. I used to have goosebumps... nay nausea before Slam finals because I was so invested. None of this is true anymore now. I watch some matches if I'm available. I still follow the actuality and watch the news. If a player I like wins a 250 or a 500 I may watch some of his matches. But that stops here. I honestly couldn't care less about the winner of Sinner/Medvedev in Australia, for instance. I watched the final and I enjoyed the match, but then I moved on and did other things and didn't think about it for the rest of the day. I watched the last 3 matches of Indian Wells but nothing will ever replace the good old Masters semifinals and finals with the Big-3. Not everyone will feel the same way as I do, but I think I'm speaking for millions of people here. People who truly loved Federer Djokovic or Nadal (or all 3, like me) will never have the same feelings for someone else. The future of this sport doesn't look bad but the excitement just isn't the same anymore. The 2000s and 2010s were the best years. (Sorry for the long post... you asked an interesting question so I wanted to share my feelings in detail)...