It would be a sad day.
The Martins would probably join other bands, I would think, but I don't know about Peter. From what I've read he's going to school and might not want to be a metal musician all his life. Mikael, though, I don't think could stop making music. He'd probably either start a new band, or more likely, start making solo records and "project bands" and such. I think we'll be hearing music from him for many many years to come... let's just hope it's with Opeth for as long as possible (or until they have run their course, I suppose).
I think Opeth would continue to be discovered by new people and continue to sell records long after their "demise". To what degree, though, I think will be largely affected by the next generation of metal bands. The more bands that come to the metal spotlight in coming years with Opeth as their primary influence, the more legendary a memory Opeth will have.