I don't think a multi cloud architecture will necessarily help this scenario. If one cloud reaches capacity, then all the users with multi cloud architecture will try to re-balance workloads to the next cloud, overwhelming it and so on. Automation will could this even worse similar to Black Friday.
Users really need to keep some on prem capacity to run critical workloads, if this is sized correctly then you get high utilisation and a lower cost than cloud. At cruch time you can choose to keep running your critical apps.
Of course you can ignore all of the above: the current round of security services, RAS, CDN, Teams, Contact Centre, telco core management, network managemnt tools etc etc etc are all being run in the main clouds (Azure, AWS and Google), so when the clouds evaporate (groan) all of the services will also stop functioning. IT apocalypse