Why would I want to develop web application in Python or Ruby while I can start with robust performance with Java and not need any switch later?

When building a web application, the priorities are: 1. Make it work 2. Make it readable 3. Make it fast. Web frameworks like Django and Rails allow developers to build very quickly. Ruby and Python were designed with readability and therefore ease of use in mind and maintenance.  Why is make it fast last? The state of technology is that good engineers are more expensive than fast hardware.  Hosting options are plentiful, whereas engineers are hard to hire.  Therefore, unless the product is expecting to blow up to Twitter scale (they famously switched from Ruby on Rails due to scaling issues), its probably cheaper and easier to add servers because your performance is slow vs. hire a new engineer because your development process is slow.

Because premature scaling is injurious to startup health. Eventually you have to re-invent your application, and when you do that you chose the appropriate language and framework.

There's a trade off. Dynamic languages are easier to work with and let you program in a more exploratory way, which is great when you are experimenting or creating prototypes.They are more succinct and flexible, they don't require type declarations and their dynamic nature makes them way more flexible. Java is one step easier to work with than c++, but it's still more low level, more verbose and requires more boilerplate code to do even the most simple task. Years ago, the standard practice for startups was to create a prototype in a dynamic language and then rewrite some parts or all their applications in a lower level language. Overtime, some startups realized that their prototypes were fast enough so they just released them, getting first to the market. They began seeing them as a competitive advantage in a market where competition is fierce and being first is paramount. Java is clearly faster, but there are countless types of applications were this performance advantage is imperceptible, since usually the bottleneck is on I/O or in the network. And actually, some dynamic languages such as Python shine in I/O performance, so sometimes it's better to use python than java. As for your doubts about whether going with Python/Ruby or Java, why choose? You can use both and get the best of both worlds. Both python and ruby have java based implementations,created to run in the java platform as well as on .Net. So you could write your applications in Jython (the java based python implementation) and, when performance matters, simply write some functions in Java. The integration is seamless. The same goes for Ironpython(the .NET based python implementation). Write everything in python and the performance sensitive parts, in c#. For Ruby you have jRuby (java based) and IronRuby (the .NET version).

Because likely the python and ruby implementations will be released into the wild far before their java counterparts. Allowing them to receive user feedback and undergo design iterations to make them more useful. Secondly not every idea is going to end up successful you need to be adaptable and agile, get your product out getting feedback ASAP . You need to be able to cut your losers without too much loss and improve and develop your winners at pace. Loosing these gains for a small performance boost doesn't resonate "Value".

I basically agree with all the other answers to this question, but I think something needs to be added. You should use the language in which you are most productive. If you have a few years of Java experience and none in Ruby then it would be stupid to use Ruby just because it is all the rage. Replace each of those languages with whatever example you want. However, that being said, a language is really meaningless, you should use the right tool for each job. Don't choose a language for premature optimization purposes nor for popularity. Your real choices are in choosing personal enjoyment and productivity, type safety, concurrency models, compiled or interpreted, etc. Scala seems cool from what I've seen but I've never used it so I would feel awful trying to build something from scratch while constantly fighting the language. For you that might be how Python would feel if you have never touched it and you have a ton of Java experience. That still doesn't mean you should write a web backend in Fortran just because you have decades of experience.

With you can have all the expressiveness you associate with Ruby, but type safety and the performance and robustness of the JVM. That's a win-win. (The bigger point being that the world isn't limited to Python, Ruby, and Java.)