Sorry to write this down but I think the developer doesn't face the reality and get the big picture here.
Tennis is one of the most popular sports in the world: TV ratings are good, it's played by a lot of people all over the world and it gets tons of press/and tv coverage on the news.
Secondly, basket games F1 or boxing/mma generate millions in sales with less popularity.
Thirdly, associating niche games with limited sales is a nonsense: a lot of "niche games" generate millions of sales.
The truth about Tennis Elbow (and tennis games) is that the whole stuff has never been properly managed.
TE is a gem with a great esport potential: huge learning curve, interesting to watch... But how many people in the world have heard about it? Money here is the perfect excuse here to justify the lack of exposure. But esport development doesn't actually require money, it requires partnerships, negotiations and contracts, stuff Managames can't or doesn't want to focus on. It prefers losing its time on a dungeon niche game that nobody cares about
Just check out the score of TE on Metacritic: it only displays 1 press review and 13 users review. How many people stream TE on Twitch? How many influencers talk about TE? The answer is simple: zero.
The audience exists though: some youtube TE videos reach 40/50K views despite the lack of exposure and the very restricted community.
At the end of the day, you can't totally blame the developer, he created the perfect tennis experience alone while teams behind Top Spin failed to reach 50% of TE's quality.
Manu is a bit like Wozniak at Apple, potentially a technical genious with limited ambition, vision, strategy and marketing skills... Just look at the website... Managames is what Apple would have been without Steve Jobs: an anonymous company that nobody cares about.
However, If Manu built a team around him, TE would be as famous as NBA 2K, we would have spectator mode, 2vs2, competitions, release on ps4/xbox... And TE would generate between 1 and 5 millions sales all over the world. The asians would be crazy about this game as well!