The decline of Twilio's position has been as pretty clear for the last 18 months now, and has been a topic of conversation longer than that. Twilio never had the margins or control of their environment to nearly the degree you need to have in order to maintain software monopoly levels of growth over time.
Most of Twilio's business has been built on reselling network access purchased via SMS consolidators. These are companies who, decades ago, got their hardware installed inside the networks of the major phone carriers. This allows them direct access to send/receive SMSs. Twilio never really tried to own the network layer and these companies continued to demand higher and higher tolls for access. Short codes are a very good example of this. Twilio spent a lot of time and money trying to sew up access to those short codes.
On top of all that, high volume customers would move directly to these consolidators. Sometimes Twilio would keep some portion of the business if the sender used a round robin model to distribute sends, but often they didn't. OpenMarket and a handful of others were the goto providers at scale.
Add on to this the overall decline of the utility of SMS. Even as SMS volumes increased, they have not increased at the same rate as messaging overall. ie: other channels like iMessage and WhatsApp continue to pull volumes.
Cue Twilio's attempt to go up market. Flex and the purchase of Sendgrid were the best examples of this. Could CPaaS work for Twilio as a business model? I think we've seen so many fits and starts with the Flex product line and the integration of Sendgrid that it seemed like perhaps buying their way in to that market wasn't panning out.
Finally, as seen in the last year, the culture just seems to have gone off the rails at some point. [1] I don't know Jeff, but my respect for him is sky high. I've read all his writing over the years and he's been hugely influential for an entire generation of founders.
Unfortunately this seems like a bit of a hasty departure. I hope it isn't, but the choice of replacement CEO screams caretaker-CEO rather than shrewd strategic move.
Another commenter mentioned Stripe [2] and how they get a lot of credit for advancing DevTools. I agree that Twilio did it first and I think paved the way. Jeff Lawson and Twilio deserve more credit. "Ask your developer" was not a small thing at all.
I also think Stripe very likely is in a very similar position, but they are earlier in their cycle and they've avoided the scrutiny of being a public company. Credit card processors are gatekeepers and fully dominate in their markets. Stripe serves a purpose for them and dramatically improves access to their networks via great tooling and abstractions, but there is still a fixed cost toll to pay and what that toll is cannot be static for interminable time. Is Stripe in the "Commerce" market like Twilio is in the "Communications" market, or are they in the "payments" market, like Twilio is in the "SMS" market? (or somesuch)
1: https://news.ycombinator.com/item?id=36382361
2: https://news.ycombinator.com/item?id=38913134