Rather than trying to deal with all the costs/conflicts and intricacies of selling their services directly to each client they are smart to empower others to handle that last mile for them. That's the true essence of a platform. When offering a platform like iOS you invite others to make money handling all the stuff that you aren't uniquely qualified to deal with and/or that isn't worth your time. I've talked to Google employees who consistently say if it's not a $100M/yr opportunity we won't even look at X.
You could argue, as I think this article does that Twilio is leaving billions of dollars on the table with this approach but they are getting something for that money... an ecosystem of fiercely loyal clients invested in their success (because companies like mine make money off of them).
Finally, does Twilio (or anyone) really know what the future of enterprise telephony will look like? I would argue they don't, and anyone with any technical experience knows how VERY dangerous + expensive it is to start building a client facing product without a clear vision for what it should be. Better that they provide the API and let other companies fight to figure out the rest... which of course we are happy to make money doing for them :)