I'd guesstimate that Twitter probably has dozens of services that are in the critical path of an average user interaction. It's hard to keep even logically optional dependencies truly optional in large scale systems involving many people.
However Twitter didn't die in the past when fail whales ruled its day, so they probably won't kill it now. It's just not that kind of business. (In contrast, a one hour outage had me directly apologizing to our largest customers on the phone). That said, Twitter can only be unstable and lack feature growth for so long before something else takes its place, so Musk is on a clock.
I don't know why this nonsense keeps being spread.
a) Steadily increasing revenue, DAUs etc and benefited immensely from COVID.
b) Were on track to being sustainably profitable.
c) Starting to get traction in stealing ad spend from Facebook who had their own issues after Apple's effects on attribution reporting.
The company was in a much better shape than many other similar SV companies around.
Well, that’s it, isn’t it? How many software systems need to keep running for Twitter to remain more or less functional?
If there are 10 critical systems that are running at four 9’s, you’d expect 3.6 hours of downtime a year, or about 90 days of uptime at a stretch if I have my math right.
If there are 100 critical systems running at 3 9’s, you’d expect 2.5 hours of downtime per day.
So yeah, all software should keep running. But it doesn’t. And something like Twitter isn’t “a software”, it’s a very large assembly of lots of different software systems and the exponential math that dependencies create.