[HarHarVeryFunny]

Sure, nothing special about sine waves as basis functions for signal decomposition. Not necessarily the best either, depending on what you want to do.

Still, as pertains to whether "the frequency domain is a real place", maybe sine waves are relevant as representing resonant frequencies of physical systems.

There also seems to be something fundamental about the way multiple radio frequencies can simultaneously propagate through a vacuum as long as they are different frequencies.