I am in the same camp of thinking that Musk overestimated how many people he could lay off while keeping Twitter going. But it seems plausible to me that Twitter was somewhat overstaffed.
How many employees are “not necessary” depends on your priorities. If you mostly want to keep a site up and running, the requirements are relatively slim. Musk also laid off a large part of the Trust and Safety team, who don’t have much to do with keeping the site running technically but were responsible for moderation. Whether their jobs were “not necessary” depends on your assessment of how much moderation Twitter needs.
How many employees are “not necessary” depends on your priorities. If you mostly want to keep a site up and running, the requirements are relatively slim. Musk also laid off a large part of the Trust and Safety team, who don’t have much to do with keeping the site running technically but were responsible for moderation. Whether their jobs were “not necessary” depends on your assessment of how much moderation Twitter needs.