I was a major customer of Twilio, among several other similar providers. Maybe if you've built a big interactive something, switching costs were high, but for what we were using them for, it wouldn't take much to switch to a new provider. Twilio's apis are nice, but most others are at least OK, and it's hard to screw up an SMS api (although GSMA has a soap standard that's pretty terrible, wouldn't have used that except that it came with some no cost messaging deals).

That said their strategy of“There is only one true strategy, build products and services for which your customers will pay you.” works, cause their service works to get calls and texts to most of the world, most of the time. If you don't do that, or the price is too high, it's easy to leave.