C extensions have been std on 4 string URBs for centuries (?).
Truth be told, going lower than E or D tends to put the general publics teeth on edge and sounds harsh on the piano and leaves a gap in the band. Gospel spends a lotta time in the cellar, but that is a unique style.
Shred metal going to low A ... no comment.
I don't know what you're trying to say about popular tastes. I mean, let's go down the list of current genres as they are commonly played today, and what that says about notes lower than E or D.
Gospel: You already said it.
Country: No matter what you might think about bro country and current pop country, fact remains that current trends favor a bass that can play low.
Pop: Obviously, pop music embraces low notes.
Rap and Hip Hop: Do I even need to talk about this?
Jazz: 5-string players, and therefore, low notes, are common.
Rock: Drop C# tuning and lower tunings are pretty common, even among bands that don't exactly have metal leanings, we're talking anywhere from Nirvana to Paramore to Jimmy Eat World.
Metal: Duh.
So what evidence is there that backs up your point that "going lower than E or D tends to put the general publics teeth on edge"? Yes, most players are 4-string players, but it seems like every genre has plenty of examples of bass notes that get lower than E and D.