Rotation won't happen for close to six months, and at this point no one knows what it will be exactly so it's too early to say. There's lots of speculation, Breakpoint/Breakthrough-on, Generations-on, Evolutions-on, and the draconian Sun Moon-on are the ones people thing may happen. If anything other than the Sun Moon-on is what happens, then yes some EX cards will still be legal. It's possible that some EX promos will still be standard legal even if Sun Moon is the cutoff, but again no one knows for sure.
That said, Expanded will still have EX cards for awhile.