Year 2038 preparations in 3.17 - why not use floating point?

What does going to 64 bit floating point give you that 64 bit seconds + nanoseconds does not?
Migrating to floating point would be far more painful than bumping an integer from 32 to 64 bits.

Being able to change the epoch at will, and being able to regularly update the clock with small increment ticks, imply storing the "master" value in some other higher-precision format, in which case you might as well just use that for everything.