SO really isn't a great example, they have considerations most companies don't - Windows and SQL Server licensing. When shit like that is involved, scale out rarely seems like a better choice.

It's not only about the amount of users, it's also a matter of availability. Even the most stupid low-use barely-does-anything internal apps at my company get deployed either to two machines or a Nomad cluster for redundancy ( across two DCs). Design for failure and all that. Failure is unlikely, but it's trivial to setup at least active-passive redundancy just in case, it will make failures much easier.