I think the proper reason why it is hard to use miles to upgrade now is due to all the miles that can be earned out there. With it seems like all carriers offering credit cards earning miles, the value of miles has gone down over the years. It is not impossible to upgrade based on miles but I think the easier and better use of the miles would be for free flights. If you would like more information about mileage programs for all carriers,
www.smartertravel.com has some pretty good articles about frequent flyer programs. BTW, they gave high marks to Alaska's Mileage Plan mostly due to the large number of partner airlines.
Hope this helps!