Sorry to hear about your loss.
Owning a woodlot is wrought with perils and like you, I am struggling with infestations myself. I am currently clear-cutting a 12 acre tree plantation suffering from a Hackmatack Bark Beetle infestation. I planted these trees in 1994 so its sad to see their demise. Like yourself I figured the kill was somewhere around 4-5 trees per acre (620 trees per acre standing), but its more like 300 trees per acre. Its sad.
But as for your dire prediction, I hope I can give you a glimmer of hope here. I can understand why looking up and seeing so many dead trees makes many think all is lost, but its not really. In my lifetime alone (34 years) Maine has been through the Spruce Budworm epidemic, a Japanese Bark Beetle, the Hackmatack Bark Beetle, White Pine Blister Rust and Hemlock Blight. We have survived them all, and in most cases it was limited to a region. (Maine) Better yet, eradicating the problem was quite simple, it just took awhile to figure out what the problem was.
I think you'll see that the Emarld Ash Borer will be limited to the mid-west. There are some really good forest pathologists working on this issue so there is light at the end of the tunnel. I know the Maine Forest Service is watching for it of course, and intense educational programs are in force here (I have a "Don't move firewood...It bugs me" bumper sticker on my car from the campaign).
As for your question regarding Ash as planking, I unfortunately have to give you my grim experience with that. It failed terribly. I had lots of left over ash lumber a few years ago and decided a deck made of free wood was better then of a deck made from ideal wood and never built; because of the cost of that ideal wood. Ash does poorly outside and my 1 inch deck boards rotted in 5 years time. Of course the wood is free, but I'm afraid planking a bridge and having it rot out in a few years time would not justify the out lay in work. Only you can decide that, but I won't build an outside project with ash ever again. It rotted extremely fast.
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"