You don't have the headcount to keep the service(s) running. You have the headcount to develop new features and services. It still remains to be seen how the reduction will effect Twitter in the long term.
These companies are generally valued as they are on the expectation of future growth. If Google said “yeah, we’re just going to keep everything as it is, forever, on a skeleton crew”, its investors would _freak out_, and rightly so.
So you beleieve twitter can maintain it's market share for prolonged period of time with zero improvement? That means it is facing zero competition?
Do you believe free market has failed?
Have changed the algorithms they used to drive engagement, serve you most interrsting tweets, make sure you are angry and depressed?
What do you think about the system they built to manage political slant, nudity, hatespeech and racism?
Or have youjust looked at their UI, and decided there are no new buttons and thus no worl was done?
Sure you have two extra SREs you don't "need", and your HR department might be slightly overstaffed. But then one critical employee gets cancer and is off work for 6 months, and another employee has to go on maternity leave, and you don't have any extra employees in that department anymore. Hope you didn't fire your bloat!