Cricket lives and dies with the India market, with all the layers which come with it. Everything else (including T20 leagues elsewhere) is unfortunately mere detail. The IPL is the centerpiece of all of it because it's found to be a sustainable business model and a product which will appeal to the (mainly Indian) masses in the long term. To
@SteveNZ 's observation that franchise T20 may not survive, that may still hold true for the other T20 leagues (see England suddenly deprioritizing The Blast in favour of The Hundred) which are otherwise basically going blindly all-in following the IPL's success, but the IPL is just too big to fail and will be protected and prioritized at all costs. Looking at it dispassionately, the BCCI have shown they capable of marketing and preserving anything when push comes to shove, they were not keen on T20 prior to India winning that first world cup in 2007. Bottom line being, for all their faults, they've shown some clear vision, however cynical, where ICC and other boards have not. Easy when you have a huge population base that continues to engage unconditionally with the game at various different levels.
In terms of numbers alone, the serious (even geeky) following for international, non-franchise cricket in India may be a mere fraction of what it is for the IPL, but as a legacy thing it is still statistically greater than the pockets of serious interest you have in places like England and Australia, which is the reason international cricket is still enough of a drawcard in India. Apart from the fact that cricket is a vehicle for nationalism here so ODI and T20 world cups are still a major thing, and so the BCCI have ensured it isn't killed off, and the ICC's main occupation now is to keep the Indian fanbase happy. They had so many opportunities to draw up a blueprint for sustaining international cricket everywhere but they've shown themselves to be completely incapable of it, and the other boards are focused on the low-hanging fruit too.
I hate to say/think it, but if the longer formats are to be revived, it'll only happen when the BCCI suddenly get creative and decide to take the lead on remodelling the World Test Championship in a meaningful way, or feel that they need a franchise test league to shake things up and promote it as a targeted product for more evolved fans. For better or worse they're the only body in the game with the political will to make things happen.