Disruptive innovation is the most cited and most abused idea in business, so let's use it correctly, because in fintech its predictions are genuinely sharp. The theory does not describe any impressive new entrant. It describes one specific mechanism: a challenger enters at the low end with a product that is worse on the dimensions incumbents compete on but good enough for over-served or unserved customers, gets rationally ignored by incumbents who can't profitably serve those segments anyway, and then climbs the performance curve until it eats the incumbents from below.
Fintech is a textbook case. The next massive payments company will not win by building a better product for Wall Street or the well-banked urban professional, because incumbents serve those customers well and will defend them like a cornered animal. It will win at the bottom: the unbanked, the micro-merchant, the gig worker, the segments incumbents can't profitably touch because their cost structures were built for richer customers. A product that is "worse" by traditional metrics, tiny transaction values, thin margins, bare-bones features, but that serves these overlooked customers acceptably, occupies ground the incumbent has zero incentive to contest. Then it moves upmarket as its capabilities improve, eventually threatening the giants who ignored it because it looked like a low-value nuisance right up until it wasn't.
The discipline the framework demands, and the reason it's so often mangled, is that most "disruption" isn't disruptive in Christensen's sense at all. A fintech attacking premium customers with a shinier product is not disrupting; it is picking a head-on sustaining-innovation fight that incumbents are structurally built to win, because they have the resources, relationships, and switching costs to swat it. True low-end disruption works precisely because the incumbent's rational response is to flee upmarket toward fatter margins, happily ceding the low end, until the low-end upstart has grown strong enough to take the high end too.
India sharpens both the opportunity and the warning. UPI democratized the rails, which means the low-end foothold is now available to literally anyone, which means the foothold itself is commoditized. That changes the game: your moat cannot be low-end access, because everyone has it. The winning low-end disruptor in Indian fintech needs a second moat, data, distribution, embedded context, that the public rails don't provide. The framework gets you to the foothold. It does not keep you there when the foothold is a national utility.