They didn't nerf them just because of the exotics. Why would they nerf hand cannon's ranged effectiveness based on the Last Word, a hand cannon whose worst attribute is range?
They're nerfing auto rifles because they're still a bit too effective at range, and they're used so much more over everything else.
They're nerfing hand cannons at range because they're currently just about as effective as scout rifles at range, which is not the intended design. Hand cannons are supposed to be for closer range precision, and scouts are for longer range.
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I'm not saying they are nerfing hand cannon range based on TLW. The reason for the range nerf is more easily attributed to the Thorn using Send It perk. Also, I don't think hand cannons should be just as viable at range as scout rifles, but why should we be pigeon-holed into using a particular weapon class if we want something at least moderately effective at a decent range?