But look I live in Hong Kong, I dont feel that way: there are other backends, we could survive without the caching for a while and Google is forbidden for 1.4bn people who get on with it very well...
Depends what you call the internet. Yes, facebook and whatsapp are gone the minute one of those 3 companies screwed up.
Bing might be able to auto-scale using Azure infrastructure but there might not be enough hardware even there.
Yes, for the majority of users the two may be the same, but "The Internet" is the IPv4 and IPv6 network on which the web runs. The equivalent companies to what you named are the transit network operators, companies like Hurricane Electric, AT&T, CenturyLink, etc. The list of transit networks is very long, much longer than the list of major web hosting companies.
Now, that said, it is absolutely true that there are ways where major outages can be the result of just one failure. Last year there was a major outage on the US east cost after a Verizon fiber optic line was severed in NYC; it turns out that this line carried many supposedly redundant links, all bundled into a single cable. The failure of that line then caused a cascade of failures as rerouted traffic began overwhelming other systems, but in the end the outage was contained in a relatively small geographic region, albeit one with many Internet users.