StackOverflow is useful because it reminds people that machines are fast, and you probably don't need that many of them to scale to large sizes. StackOverflow is used by the entire global population of developers more or less and it runs off of one large MS SQL Server + some web servers. No auto scaling (not enough hw expense to be worth it), no need for fancy cloud LBs etc.
Sure maybe your service is gonna scale to more than the world's population of developers. Great. But ... a lot of services won't even go that far. For them it's hard to conclude it's really needed.
To pull off a StackOverflow you do need skilled sysadmins though. As the article points out, a surprisingly large number of people who call themselves devops don't really know UNIX sysadmin anymore.
Changing capacity automatically always has the potential to backfire. And as AWS & Co need to keep those servers running during times of less demand there's no way it's cheaper unless you have really unusual traffic patterns (even then it's probably not).
SO's read to write ratio is enormous. While they talk about a handful of different bare metal servers a vast majority of their hits are handled by their caching layer which is not one of their bare metal machines.
Their write loads are not explicitly time sensitive either, if there's a delay between a successful post and cache invalidation it's not a big deal for a majority of their traffic.
Not every model is quite so forgiving. But even then there's a lot of stupid wasteful "infrastructure" made by people who think they're going to be Google, or at least tell their investors they'll be Google.