That is a reasonable argument (at least in theory). I think using the hide would be either less immoral, or not immoral at all (although not technically vegan).
A possible argument against would be that it might be distasteful or direspectful to the animal involved. I mean, would you like somone wearing your skin after you die? And if you think it's fundamentally wrong to wear human skin, why is it fundamentally not wrong at all to wear animal skin? Well I don't know for sure that answers to that and I don't think it's important, but well just giving some food for thought. I guess personally I wouldn't do it but I don''t mind if others do.
Another argument would be that people would see you wearing animal fur but not understand it had died of natural causes and it would be very confusing if you say you were vegan. If the hide were used not for clothes but something else that could be an issue.
Now, in practice, where are you going to find dead animal hides? As a commercial enterprise, it doesn't seem at first glance to make sense and an animal that has just died make have poor quality hide or even diseased.