Lets say there is a Switch game this year.
That would indicate that next year is a break year for GF because they never released 4 years in a row (which is understandable).

Now lets imagine said 2018 game is Kanto remake as some rumours suggest. That would mean gen 8 wouldnt start until 2020 and the remaining two years would be still gen 7.

So that means that for another two years there would be no new creatures to promote (and most gen 7 mons already have been promoted). Even if the rumoured Kanto game would have some new mons like USUM then once again they would most likely be just end game legends and even thats not all that likely

Either way it would be a lot better to get gen 8 IF we're getting new game this year. Otherwise it will be just more old stuf for the next two years