You're underestimating the aerobic ability one needs to run two 50's back to back. It is incredibly difficult (obviously, since it's never been done before). Fast 400 meter guys can't move up and do it, they don't have the strength. Dudes like Merritt and Wariner would be gassed after 550-600 meters and would look run like crap coming down the homestretch. I doubt either man could break 1:47.
The argument that a great runner in once distance could easily "move up" and break the record in the next distance sounds good in theory, but you're forgetting that different events require different types of training. By using your argument, couldn't we say that the mile record is weak? Why couldn't a 1:41.11 guy like Kipketer move up to the mile and run a 3:40? I mean, two 1:49 800's would be EASY for him right? Or why couldn't El G destroy the world record in the 5000? Surely he could run 3 sub-4 minute miles in a row! Except not... because different events require different types of specialized training.
Someone will run faster than 1:41, maybe soon. It will likely be a Kenyan. It will be someone who has great natural sprint speed, and has build up a solid aerobic base as a youth by running 8k/10k races. It will be someone similar to Asbel Kiprop, who was the world junior XC champ and has run 1:43. The record will NOT be broken by some 43-second 400 runner, who decides to move up.