Sure, you can run the platform with 1/10 headcount with significantly degraded user experiences (say ~98%). This is not a problem for startups but people usually have higher expectations for established companies. As always, the last 2% is a hard problem and business doesn't really want to deal with a such unreliable platform. You wanna onboard big advertisers which potentially spend $100M ARR? Then you need to assign a dedicated account manager to handle all customer escalations. PMs then triage and plan their feature requests and later engineers implement it. Which all adds up.
And they also uses your competitor's product, like Google, FB, TikTok etc etc... Twitter is a severely underdog here, so you need to support at least a minimal, essential subset of features in those products to convince them to spend their money on Twitter. That alone takes hundreds of engineers, data scientists and PM thanks to modern ad serving stacks with massive complexity.
Yeah, it ultimately boils down into a simple fact that it's really hard to take other folk's money. You need to first earn trust from them. They want to see if your product is capable of following a modern standard of digital ad serving for now and foreseeable futures. Twitter has spent lots of time for earning trusts and the original post is one evidence of such efforts. And this usually needs more man power. You might be able to do that in a more efficient manner, but I don't think that's as simple as firing 75% of your entire headcount.
