The documentation on the eBay developers site suggests that everything is transitioning to RESTful APIs. I think eBay could do a lot better job making sure the RESTful APIs are all working as expected. I have found inconsistencies between the sandbox and the production environment working through the code to list an item and publish an offer using the Sell APIs. I have clients who are eager for me to integrate a "Sell on eBay" function within their eCommerce platform. Even if the Trading API is working, it's a different form of EDI and requires different code, and I don't want to have to write code twice if the older, XML based format is being phased out.
My understanding is that it's eBay's intention to continue supporting their older API families (like Trading API) indefinitely. This was announced at their eBay Connect Dev conference last year, which I attended. I mean, nothing's certain of course. But that seems to be their plan.