Good points. I suppose if both Canon and Nikon were to quit developing DSLRs that would leave users with little choice. But, they would still have to weigh their ability to get customers to migrate vs. losing customers who simply choose not to buy into the mirrorless market.
Would DSLR users, who skew older, simply hang on to their existing bodies or drop photography as a hobby altogether? I'm in my mid-sixties. I'm good for probably another generation of professional level cameras, but after I retire in a couple of years, and quit doing any professional photography, I don't know what choices I will make; and I'm a serious GAS addict.
Certainly there would be some potential 5DV buyers who, if they don't have that option, choose instead to keep their money in their wallet or spend it in other ways.
Of course, you are correct in that a lot depends on how good the improved EVF is, but I'm not sure that is the only factor. I come down squarely on the side of Canon wanting to hedge their bets. Especially because the roadmap is pretty clear and the marginal improvements likely to show up in a 5DV already exist in other models, so the incremental costs may not be that high. Only time will tell if I'm right or wrong.