People smile at me when I tell them that I am still developing in Visual Basic 5 - Maybe you wonder also. (For the case you wonder why I did never do the change to VB 6: It was because of the IDE stability especially when using threads and other lower level Windows API - VB 5 was the better thing here which is another prove that new is not always better. ;-) )
Well, I learned more than 10 programming languages yet but I could hardly say that I can be very productive and efficient with all of them. It is one thing knowing the syntax, a second to know well the standard libraries, a third to know the libraries which are available out there from 3rd parties and a fourth of having your own libraries for the recurring jobs in your realm of work. Further it makes a big difference if you are using a language every day or only occasionally. In general it is better if you have one language that covers all your requirements (e.g. can be used for thick clients and web applications).
It takes years until a system has evolved and reaches it's maximum performance. What for me applies to the programming language used, applies for many other IT infrastructures also.
Let's say, you think of switching from Microsoft Office to Open Office. There will be some migration work and maybe a lot of annoying details (apart from macros that have to be rewritten) that will reduce productivity of the employees in the first step. And therefore you will not do it until it is really worth it.
IT changes fast but this does not mean that you have to be always the early adopter. The primary question should be the costs and the benefit in the long run.
There will be the time when the benefits of a particular change legitimate either big investments (of time and money). In my case switching to Java brings independence from the underlying operating system used for instance, easier network programming, better IDE support when developing web applications and other advantages. However it will take a while until I will reach the same productivity level and development speed as I have in VB where I can reuse libraries that I developed in the last ten years.
Not always big effort is needed for implementing new software products. Just remember that when you start investing more time into a particular product then it should be one that will be used a longer time and make sure that you use it efficiently.
Related posts: IT project costs explosion, The Open Source movement, Ignorance of the different.