All this puffed up pomposity around opera or classical we currently deal with is much different than in the "hey-days".
If you went to a nice restaurant and I came over and started talking loudly with spit flying into your dish or took some food from your plate it would ruin your enjoyment. When we're trying to hear the subtleties of interpretation from the performers, the faintest sound detracts from that.
I think most people can accept that talking in a cinema or using your phone is pretty anti social so I don't know why classical music gets such a bad rap for being pretentious when it has a conduct. The only thing that drives me crazy is people always seem to cough when the music slows down and gets quiet. Please do the exact opposite. Don't spoil those tranquil serene moments with your coughing. Cough when the percussion and the tubas are storming and no one will hear.
That said, I'm all for bawdy music experiences - I once saw a tiny production of Le Docteur Miracle in a bar in London which was extremely fun (by Popup Opera UK) and if you go to Sarastro's by Covent Garden you'll get some fun rowdy opera hits sung at you while you dine. Plenty of outdoor picnic concerts are similarly laid back
As to clapping: As late as the late 1800s, it was okay to clap during the loud parts of the song. Baroque music also often has long cadenzas that end in a big trill, which was a signal for the crowd to go wild. I agree about getting less strict about this, and historically things were less strict. At the same time, I recently saw a performance of Tchaikovsky 6 where the audience started clapping between the last two movements (a triumphal march followed by a tragic final movement), and a large part of the energy of the piece was lost from the audience doing this. I think this rule mostly comes from the audience not understanding when not to clap.
Also, lots of classical music is done in environments that aren't concert halls and these are much looser about the etiquette. A concert hall is a nice place to go, and many people go on dates at concert halls, and a lot of the vibe for the audience comes from the fact that people dress up for the occasion.