I have a big winforms app which I now rewrite as an HTML5 app for the sake of portability.
There is an important UI component with a lot of logic and BCL usage that will be very hard to rewrite as HTML/JS. I am thinking to have this component only in SL.
Looking 2 years ahead does this still give me portability?
The moonlight project seems stuck. Apple may decide to have new rules or break some compatability.
Are these real risks or am I expected to have at least what I have today?
There's no absolute answer to this of course, it's purely opinion.
However, my advice is to stick to general standards as much as possible; as you say - SL may deprecate, Moonlight might not update again, Apple may completely drop flash support. Who knows ?
They are real risks, but you can program defensively - document your API thoroughly, consider writing a REST/WCF/SOAP interface that can provide the behaviour your program needs.
You're right to be concerned that there are risks basically!