Of course its the OEMs and carriers who are to blame. But a two month rollout isn't that bad. MS is trying to avoid OEMs and carriers by providing the preview. Also, I think Samsung and HTC released gdr3 without any firmware, so there was no waiting even for those who didn't download the dev preview. That eliminates a lot of fragmentation.
while you are waiting for black for your 620, there are carrier locked 925s which just got the update last week, almost 6 weeks after the rollout began. Another kudos to MS for dev preview, they really did a good thing bringing that to the users, so that the main OS updates are available soon.
Also, I doubt that there will be "delayed" releases after the acquisition of Nokia is complete.
Try suing a better internet connection. You seem to have a lot of problem downloading updates.