As someone else has already said, marathon courses vary greatly. Has anyone said "ah no sorry, a record set at (for example) London can't stand because it's flatter than Boston?"
No of course not - courses vary, altitude varies, even the weather makes a difference where a world record is concerned and it may come down to a matter of seconds.
Obviously in the absence of cheating, once you start qualifying world records in this fashion you may as well tell everyone to run as fast as they can for 26.2 miles on a treadmill. Why don't they do that now? Because running a marathon is special, and doing that would suck the joy out of it.
The big name marathons are amongst the few occasions where uber-plodders like me get to run "with" the world's most amazing and talented athletes. They draw big crowds and big sponsorship for good reason. PR was on flying form in 2003 and she delivered. To downgrade her achievement now is a disgrace.