Sumo for Japanese whether at home or living abroad is usually one of casual interest and while they are as patriotic as anyone, with no Japanese Yokozuna despite the brief Kisenosato period when interest was up, generally it is a topic not on most people's lips. The casual fan base has certainly risen according to NHK perhaps with the introduction of highlights and 3 days of live per basho.
It looks like sumo will enter a very lean period. Uncertainty about Terunofuj's longevity and an apparent lack of strong opposition for him means quality and strong numbers may be rare. If Teru stays relatively healthy he should dominate. We may be waiting 2 years before it rebounds, perhaps more.