Yep - I guess it depends on how quickly you can commission a new server / how much down-time you can afford. However I would still say the same - if budget does not allow it, just do it. In many non-critical applications, being down for a few hours or a day shouldn’t be too much of an issue. It’s like @hassan says below - I think it’s better than overspending.
I was going by the OP’s 2,000 to 3,000 online users, where I think a dedicated box would be cheaper. Having said that, dedicated servers have come down quite a bit in recent years so I don’t imagine the difference to be as significant as some might think (unless your app is pretty small).
It would simply mean switching to a different (cheaper server). Most rental contracts are month by month. I wouldn’t buy a server (i.e. co-locate) unless I needed more than 10 servers.
_For anyone interested, there are some good recommendations (and further discussion) on dedicated servers in this thread: https://elixirforum.com/t/deploying-elixir-opinions-on-heroku-digitalocean-bluemix-etc/289_
