[papaf]

it routinely eats up close to 4 GB of RAM, which in my case is an entire VPSes worth and costs me about 60 Euros a year

You are absolutely correct that running other peoples Ruby code is expensive. I would argue similarly for Java because of the high RAM requirements.

However, if you are a company developing your own software, if you use a more productive technology and it spares just 1 programmer then you have paid for a few hundred VMs and running costs become insignificant compared to labour costs.

This is an argument in favour for both Rails and the Java monsters.