Wie lange noch relevant...?

Not so sure that the future is only FF and m43. Mirrorless APS-C will survive. Why should Sony abandon their successful line of NEX models? But APS-C dslrs will disappear quite soon. In a close future we will see dslrs only in FF format (and bigger!) Canon and Nikon have both introduced FF cameras aimed at amateurs, 6D and D600, and I believe there will be even simpler and cheaper ones of that kind.

And soon enough we will see new mirrorless FF models as well!