EPUB 2 will probably not remain an active standard, in the sense that it would evolve over time. The EPUB 2 standard is of course there and stays there and there will be files and devices that follow that standard.
Looking at the differences between EPUB 3 and EPUB 2 it seems entirely feasible to make an EPUB 3 device (with or without multimedia) that also properly supports EPUB 2 files. This is little extra work and much more interesting for device manufacturers than asking for and implementing EPUB 2 extensions.
There is probably also going to be a subset of devices that don't support the EPUB 3 multimedia extensions, but are based on a rendering engine that supports EPUB 3. Whether you depict such a device as EPUB 2 with some EPUB 3 support or EPUB 3 with backwards compatibility is probably a marketing decision.
From a file generation point of view, you can include EPUB 2 compatible info in an EPUB 3 file. I expect publishers to do that. And if they don't it is feasible to add that information later on to such EPUB 3 file. I expect e.g. Calibre to do get such a feature, if it not already can do so. Of course that will not give you multimedia, but for normal books (with the same info as EPUB 2 files have now) that will work.