I submit that this is an overly simplistic view of what "the infrastructure" is, a view common among those who write software: the infrastructre is anything that isn't the code they write.

There are very few scenarios where a non-trivial site can be kept up (that is, not "dysfunctional or down" as the OP mentions early on) with merely application code and generic, undifferentiated, off-the-rack infrastructure.

To scale even modestly, one must customize. The customization can start with the web framework and can eventually lead to hardware components (though the "lowest" I know of is Google's custom mobos). There are also operational considerations, such as

I think there are two separate questions for any enterprise. The first is, is further customization an optimization. The second is, is the optimization premature?

To suggest that, for all startups, the answer to the first is always "no" and/or the answer to the second is always "yes" would be overly broad.

For the folks that have created many web apps many of these questions have default answers based on experience but for a first timer like me I can't afford to learn by failing at every fork. I need to minimize my failed choices by searching for external input from people that have been through it before.