Just suppose...

What if bitcoin seems to average about 7500 USD or so over a very long period, like a year or even more. Currently, this isn't high enough of a value in USD to really mine anymore with GPU's, if mining for profit in USD anyways. I do not mine with ASIC miners and I really know very little about them, even though I almost bought a Bitmain Antiminer S9 before I build my 8 GPU rig.

Will mining taper off to little to nothing, will difficulty levels be adjusted to compensate miners, or what else?

I would be interested to know some thinking on this, should it become a reality or even close to a reality for some length of time.