You can test all you want, but spikes in demand can still take you down. I dont care how big your VPS is, there will be a point that it can't take anymore, and in real life, that point will probably be sooner than your testing will predict.

I repeat, infrastructure problems will kick you when you least need it.

You have a point about lock in, but there are many cloud providers that just supply hosted versions of open source software (amazon RDS, mongo HQ) so you can still use the cloud, but have the ability to take your data away with you.