Developers (myself included) worry too much about future unlikely pain and suffering, especially the degree of said suffering. Maybe you spend a few weeks here and there rewriting things. Big Deal.

Scalability and performance is complicated. Unless you KNOW your product will have a big splash, premature optimization will kill your productivity. And you will get it wrong. You will get the implementation wrong. You will optimize the wrong things and not really understand your bottlenecks. Especially if you've never scaled anything before and haven't been bitten twice by all the compromises you have to make.

Distributed systems are hard. Multi-threading is hard. Sharding is hard. CAP is a bitch. If you can scale vertically, do it. Avoid the demons of distributed work until you require them.

Most of the services/apps we build today would do just fine with setups like Mailinator or gasp ACID-compliant data stores.