It seems like you and your users are quite pleased with your current Silverlight application. Silverlight will be supported for many years to come, and Telerik is also heavily invested in this technology. So there is definitely no reason to rush out and replace an application that you and your users love.
I also understand your concern over the growing tablet market. What I would recommend for you is to see what functionality is most popular in your Silverlight application (you can use a product like Telerik's EQATEC
to discover this), and slowly expose functionality bit-by-bit as required or requested from your user base to HTML5 (this can be done with the help of RadControls for ASP.Net Ajax
as well as Kendo UI
- both of these are solid products and are ready for prime-time). What is also nice about this approach is that you will be able to reuse your existing business logic code.
I hope this recommendation helps you!
Developer Evangelist, Telerik