Yep, Diablo is a CRPG. A piss poor and boring CRPG but a CRPG nonetheless.
If the genre evolves, which it has over the last 20 years of CRPGs then all is good, but not all games takes a step forward and sometimes a game hits an evolutionary dead end. For basis of comparison I judge older games by doing a comparison to games that were released in the same timeframe (plus or minus a year or two).
I judge CRPG Hybrids as CRPGs, except for Dues Ex. For some odd reason that is the only exception. Can't explain why, but it is.