If everything were expanding with the Big Bang expansion, stars from each other , planetary systems, atomic systems, nuclear systems there would be no possibility of measuring the expansion, because the units scientists use to measure would also be changing (without our being able to perceive it) following the expansion.
The hypothesis that allows to measure an expansion is that the bound states of the four known forces, cannot be affected by the expansion. This allows to treat galaxies, and the stars within them, as gravitational bound states ,not changing size, and allows for expansion and the infrared shifts one measures.
So it is not a matter of consensus and beliefs, it is an inevitable axiom in the model, the Big Bang, that fits the data observed. At the moment it is validated by all the known observations. Thus the hypothesis is extended to the bound states of the other three forces, which bind more strongly than gravity.
Maybe future observations and measurements might show discrepancies with this hypothesis, then a modified Big Bang would have to be used , though I think the argument of "how one could observe any change when the units change" will have to be addressed.