It may be bumpy in the short term, but then all of the supremely confident naysayers are going to look ridiculous and spiteful if it turns out that Twitter can operate with 80-90% fewer employees.
The fact that a site that big hasn't broken yet shows how good they actually were and in the end he did fire good engineers while not even tackling Twitter main problems which none were technical. Bots are still rampant, moderation is gone, monetization is non existant and advertisers were scared away.
Unless things change what you're seeing is not a success, it's a zombie.
Institutional knowledge is necessary to sufficiently onboard any new hire, without that you have headless chickens running around trying to figure out what exactly are they working with. How do you solve Chesterton's Fence when there's no one around to tell you why the Fence is there?
Any engineers capable enough to rescue this trainwreck will likely be able to secure better compensation elsewhere without having to endure Musk's new indefinite "hardcore" death march.
Twitter is going to operate on a skeleton staff of H1B's dependent on their employment for their visas (who are probably already floating their resumes) and whatever underqualified graduates they can hire.