Easton, among others, killed City Center, not downtown (downtown was already dead before Easton). And Grandview Ave certainly is not City Center. I'd consider it more like a tiny Short North, which last I checked, still seems to be humming along despite Easton, Polaris, and Tuttle.
I'd tend to agree with CMH_Downtown on this one. With the exception of the Panera, everything on the Grandview Heights part of Grandview Ave. is a local business, and not apt to be duplicated at Grandview Yard. I'd expect those businesses will be able to co-exist with the ones at GY, which I assume will wind up being mostly national chains. And really, most of the places on Grandview Ave. are specialty shops or destination places and not like a drug store or grocery store where you'd want to get in and out quickly and thus convenience of parking would be a big factor in deciding to go there or not. So I would guess people will continue to go to Grandview Ave. and that strip won't be overly damaged by GY.
I'd tend to think the Shoppes on Lane Ave. and maybe Lennox (although they have a fairly captive audience of OSU students there) will be more threatened by Grandview Yard, since they're more likely to have some overlap in terms of who sets up shop there. I could see people who have been driving to those places from Grandview or even VV/HW deciding to go to GY instead because it's closer. And others going just because it's the new place.