Let's be frank - constantly allowing the players to ascend old weapons will create no incentives whatsoever to obtain the new and fresh gear the development team creates. Eventually people would find the holy grail of weaponry and Bungie would be forced to either make a better weapon, over and over, therein creating endless power creep in the meta, or they'd be constantly creating inferior weapons that nobody will ever use unless they're too new to the game to get that "holy grail", which therein bars them from obtaining "top tier" gear. I like my Fatebringer as much as the next guy, but you must acknowledge that they don't want one weapon to reign supreme for the entirety of Destiny's lifetime. Why bother making Arc primaries if everyone actively ignores them? Tl;dr - Allowing upgrades results in power creep and unbalance among the PvP community, or the creation of sub-par weapons in comparison.