I agree with the first half of your statement, the Testastretta engine will be in the SS for a while. The SS always had the previous generation engine.I doubt the testastretta engine will be replaced anytime soon, by soon I mean the next ten years. Only the top of the range panigale will be getting the v4, the base models and mid range will continue to get the present engine.It comes down to the world superbike changes to the rules governing engine capacity. So they have to adopt a derivative of the motogp engine so that a 1000cc superbike can compete in WSBK. To compete in WSBK, the same bike must be in production and for sale to the general public.
It's possible you have misunderstood, for ducati to compete in the world superbikes - WSBK, they have to use a 1000cc engine from 2018, so they are taking technology from the motogp engine, not the whole engine but the technology of that engine and putting that into the top of the range panigale. WSBK is massive for promoting production bikes, and if the bike is not in production and sold publicly then it may not enter the championship.I agree with the first half of your statement, the Testastretta engine will be in the SS for a while. The SS always had the previous generation engine.
I have doubts about the second half - to compete in MotoGP the bike must be identified as wholly unique to any of the manufacturers production bikes. Will the new V4 engine be seen as enough of a derivative to compete in both segments? Will there be a new MotoGP engine in 2019?