The aspect here is that we have zero evidence that the next breakthrough would allow the cracking of 2048 bit RSA and not, say, 4096 bit RSA. Chances are, any such breakthrough would allow a more general solution. See quantum computing for an example where this would be true.

The inherent uncertainty of cryptography seems to cause people to want to do something, anything, in an attempt to decrease the uncertainty, even if there is no rational reason behind the approach.