2 generations both smaller than 100 isn't normal either.But hasn't been a thing ever since. It's not the normal by a long shot.
I mean, Rock types aren't that common in general.. if you're asking for 2 fossil lines per generation when each generation is only 75ish in size... might as well rename Rock type to Fossil type as fossils outnumber actual current-day Rock Pokemon.
So if you cannot get the possibility of no fossils in your head for some reason...try thinking of it this way. XY had fossils, so now SM has Lycanroc and co. instead.
By the looks of it, we have 70+80=150 with XYSM, so it still works out as "normal" in that sense?