how far down did society go when the Roman Empire fell in the west

We can make the argument that for the average working stiff (insofar as there ever is any such figure), quality of life went up in some cases. It is best not to fall into the trap, so commonly seen in popular comparisons of Imperial Rome versus Medieval Europe, of looking to elites of the former versus peasants of the latter. I'd certainly contend that, past 1000 AD, your average Medieval town dweller has equal access to fresh water from the local aqueduct as their Roman analogue.

• u/BRIStoneman has a short but most illustrative comparison of Rome versus the Medieval Period;
• and on why we don't do 'Dark Ages' anymore;
• and u/dromio5 and u/wickie1221 also address inventions in the Medieval Period, in addition to why we don't do 'Dark Ages'.
• and I will also provide you with my Middle Ages Are Best Ages Compilation, which does not directly compare Rome versus Medieval Europe, but highlights multiple things that the Medievals did have.

For more linkage, see next post because tag limit.