Borrowed power is any ability that is a feature of a single expansion and which you have to build up over the course of the expansion, which is then allowed to die or is removed after the expansion is over. Which is the most likely fate of dragonriding. It most likely will never be expanded beyond the dragonflight zones and the dragonriding mount you put all that effort into will probably never be usable outside those zones.
I will wait and see, I have lots of other things I can do without buying this expansion until then. If it ends up being interesting, I can always buy it later. I don’t raid anymore so being in the main wave going through is irrelevant to me.