Musk wants to save $1B from infrastructure costs, so the service quality will very likely degrade. The current business focus is money saving at all cost so I don't think SRE or whatever engineers are not very safe from layoffs.
Services that are on the hot path of serving requests and generally keeping the site up and running are a tiny, tiny fraction of overall infra costs at companies like Twitter. The majority of spending is on storage and processing of logs, metrics and analytics data. Now you can argue that Twitter will be worse without this data, but it isn't as simple as "Twitter will go down more if you cut infra costs".
I don't think you understand what you're talking about or Twitter's business model. Those log and metric, analytics are the reason for advertisers paying Twitter. What's the point of saving $1B if you lose $2~3B of advertising revenue?
Oh and good luck with cutting dependencies on those storage while firing all those engineers with knowledge on those infrastructure. From my experience, migration and deprecation is usually much harder (I would say 2~3x difficulty) than launching a new stuff especially in an established big company.