I would actually be slightly worried playing with Johnny Football for another season in terms of my draft potential. What if scouts end up perceiving him being a product of Manziel's success and uncanny abilities? I agree with you though that there is no guarantee of him being a top 5 pick next year.
But also when you see guys like this stay usually it just means they were never 1st round picks to begin with. How many times have we seen "top 15" picks stay another year and drop out of the first round entirely? Haven't watched much tape of Matthews so i don't know if thats the case, but usually staying is the first indication.