There's no reason it wouldn't fruit, given time. The graft is the same age as the tree it came from. The rootstock is a seedling and needs about 7 or 8 years to fruit. You'd still want to shape the tree as it grows. I'm a long way from your zone, but all the same, I have a tree that lost the graft. It's sent out sprawling branches in all directions and is a bit of a nuisance. I keep telling myself I either have to cut it out completely or cut off all the lower branches (when I have time).

I have tried a few Mangoes over the years...and every one wanted to go vertical more then horizontal. I tried two type of 'Manila' (same name but the two trees looked very different) and 'Baileys Marvel' that had been like a rocket in growth-- and form. I tip pruned it one year..and it sent new tips straight up.
What I have now is the Haden rootstock. I also now cover it on any low 30's winter nights. Makes a huge difference it turns out in our marginal to the extreme climate.

My "nuisance" one has branches going up, and all other directions as well. Years back I cut it back but never got around to removing the stump. Forgot about it except when it got in the way. This is what it looks like today.

Just curious Stanofh, I have a Nam Doc Mai (grafted) and I live in Orange county, California....Question on the hardiness of the tree: I think the temps got down to freezing to near freezing this winter and I had the tree unprotected. Is it odd that there was no damage to the tree (3 feet tall)? I saw some light frost on a neighbors roof and thought it may have been near freezing~I don't know. This got me thinking about temps after I read your post about losing part of your tree at 32 degrees. Thanks! Best regards, Chris