They're a SaaS play with no lock-in. Which is great for businesses who want to use Twilio on a small scale then grow strategically. Sucks for Twilio because they want to have as much predictable revenue as possible.
Until then they are the first choice for a large portion of the developer population.
After that, it's just a numbers game: will it cost more to redevelop the functionality they provide (either on your own or with another vendor) than you will save over a reasonable timeframe? If the answer is yes, you keep paying twilio, otherwise out migrate.
I have no first hand experience on what size makes sense for twilio, but this article gives us an upper bound (a huge one, to be sure).