I'm a developer who worked with distributed systems so I know what that would entail. Some alternative services would quickly step in. For Gmail, the gmail.com domain could easily be bought up and redirected to another provider overnight. Amazon might require a bit more time, but another company would quickly buy their data centers and software (or use open source cloud hosting software; there are many to choose from)... The data centers would be operational within couple of weeks (possibly running different software). In the meantime people would find alternative hosting providers; now with Docker and Kubernetes, it's a lot easier to switch providers - Many software providers which didn't build 'cloud native' systems (or at least those who didn’t care about vendor lock-in) would go out of business (which would be a good thing) - It would create opportunities for new, more competent and capital-efficient providers to replace them (maybe those new ones will actually be mindful about technical lock-in).

All businesses are replaceable, the only thing that would change would be that the assets would have new owners.

Anyway my point is that none of these companies are more useful than cryptocurrencies. They are mostly about hoarding customers, locking them in and leveraging network effects. They add very little value. I use AWS myself and it's a great service but if it was shut down tomorrow, I could easily find another platform. There might be a temporary increase in hosting costs as people would migrate to other providers. It would be an inconvenience but wouldn't have any lasting effect.