The major tech companies have done their best to embed themselves in the web's infrastructure, but any one of these organizations could deteriorate and we wouldn't lose the internet. Yes, even Amazon and Google.
Would argue with this point though - I think a few of them are almost too big to fail, given things like intranet services, emails etc making up the internal infrastructure of many critical companies, and even in the event of a messy collapse being avoided and such services continuing, the sheer amount of personal data that would be essentially sold off and scattered to the wind makes a bankrupt alphabet a very frightening prospect.