I think CMS is widely used and will still work some more time. Anyway, G1 is supposed to replace it sooner or later but the "battle calculator" creates huge objects, causing humongous allocations, which can be quite harmful. I tried with 6 MB regions, they still happen. All java apps using large objects are not good candidates for G1 migration, especially when the CMS works fine. I have the same problem using apache jmeter on HTTP content. That's only my opinion, of course.
Note also this :
"This JEP does not specify the major release in which support for CMS will be dropped. The decision of when to do that will be informed by the extent to which the G1 collector proves to be a suitable replacement for CMS. In the meantime, CMS users are encouraged to migrate to the G1 collector (-XX:+UseG1GC)"