1. Any financial advisor who recommended to these startups that they should keep >250k in a regular bank account should be fired. It's totally possible (and regularly done) to spread out cash among several financial institutions to protect against this very issue.
2. Any regular account with two or more signers (very typical for a business account) is insured up to 500k.
3. If spreading out your 6- or 7-figure assets to multiple institutions is too much of a burden, literally every business bank has special accounts or add-on features that either raise the FDIC default limit of 250k, or supplement it with external insurance. Again, if any startup's financial handlers didn't recommend this: fire them because they entirely failed to do their job.