Why would Low Budget full frame buyers buy High-Dollar L-glass? Seems likely they would buy a 5D3 if they had L-glass money.
Not necessarily. I own a 70-200mm f/2.8 IS II and a Rebel T3i. Bought the 70-200mm practically new on CL for $1700 with intent to sell it for more, but decided not to after seeing how FREAKING AWESOME it is.
The 5D3 is waaay too expensive but a sub-$2000 full frame is of great interest to me, whether it's a new body or a price-dropped MK2. Anything I can use to take advantage of my L glass without hitting my wallet too hard.