Sumo is most analogous to baseball in America, a national past time with an aging fan base and decreasing youth interest. Like baseball, a lot of grey heads at games and events. It also has religious-cultural aspects not found in any American or European sports (maybe bullfighting aside, though they call themselves an art not sport).
It's also being dominated by Mongolians. This new yokozuna is the nephew of another mongolian yokozuna who along with a fellow mongolian dominated the sport in the 00s and 10s.
The last ethnic Japanese super champion, Takanohana, was reigning all the way back in the 90s. So you'd have to be well into your 40s to remember a great Japanese champion.
I think until Japan produces another Takanohana, Chiyonofuji, Kitnoumi, or Taiho like wrestler, the sport will continue losing popularity (maybe long term even a great champion wont save it from deeper rooted trends).