To some extent i agree; most will never scale so why worry about it? X times out of Y you'll come out ahead by just ploughing on and not considering the future. Even if we are thinking up front, we can rationalise that it's just a gamble, often one worth taking!

It definitely doesn't take Facebook sized infra to outgrow a technology though. What if the gamble doesn't pay off? What if you planned to scale the central NFS server dependency by just adding an extra NFS server but have now found there's no rack space left / no budget / a purchasing delay / insufficient network capacity / cooling capacity / power capacity / a.n. other unplanned problem.

For the SSH question, i couldn't reliably get more than 250 concurrent connections outbound from one circa 2008 blade server. From memory that would have had a spec of dual core CPU, maybe around 2.4Ghz with 8Gb ram using PAE as it would have been a 32bit kernel (our spec, the cores will have been 64bit). They were multiply-connected at chassis level on myrinet fabric in one DC and infiniband in another and the resource being exhausted was CPU.

These days all the blade servers are gone but we see an absolute explosion of virtual machines so it's a similar and still relevant problem in many ways.