How much cost in labor, disruption and time do you think upgrading every backbone router would be?

When would you have done it? When the hardware was 5 years old? 10 years old? I've been on change operations where the hardware was over a decade old and our biggest concern was making sure we had replacement PSUs on hand since the thermal shock of a reboot might take them out.

This is the problem, you have absolutely no idea what anything costs or how it works - you're simply thinking "lifting those bits sounds heavy, what if we made them lighter?" as though a committee of engineers, designers and industry didn't spend a lot of time considering exactly that problem.

If you could add 1 byte in software and be done with it, it would have already happened.