The short answer is, nothing. The single most important aspect of a bike is "does it fit?" And clearly, yours does. The only reason to buy a new bike is because you want one. A new one won't make you any faster, but it might make you happy.
For reference: my grandfather had a custom steel frame made for him in Chicago by Oscar Wastyn in the late 1930s (Wastyn would later go on to design the Schwinn Paramount.) My grandfather rode that bike until his death, almost 60 years later.
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