All the interstellar gas is lost from both galaxies. Many of the stars within the galaxies collide and go supernova. A G main sequence star, an M main-sequence star, an RR Lyrae star, and a Cepheid variable star all have the same apparent magnitude. Consider a quasar several billion parsecs away. RR Lyr variables stars cannot be used to measure the Hubble Constant.
Astronomers have difficulty studying the structure of the Milky Way Galaxy.