What I'm curious about is why it is a single-PaaS; I'd have expected Salesforce to have the customers quite isolated so the chance of bringing down multiple customers at once was much smaller.
Also, mono-tenant architectures is no golden bullet either. Such architecture (often coming from a formerly on-premise product that was SaaS-ified) can easily become hell to operate as it multiplies the integration points (DB parameters, URLs, allowlists, etc).
It's also quite wasteful in terms of resource utilization and hosting costs.