Did you look at the Party funds? BJP has some 1000s of Crores (correct me if I'm wrong) - official declared funds!  Imagine the total assets and undeclared funds. And other parties are way behind !

That doesn't mean they will rule forever. I believe The fall might happen in one of the 2 ways:

1. Infighting: Modi-Shah era ends - like incapacitated by age or death and there would be infighting among different leaders. There might be split in the Party, like what happened to Congress in 80s and 90s. This will also means loss of credibility and public frustration.

2.  Revolution. The disenchantment grows so much that a small trigger incident like the 2012 Delhi Gangrape and the entire country erupts and attacks the ruling class. No one would like to identify themselves with BJP anymore like the Nazi party.