No offense, oldman, but my most recent flight was on United (LAX-BOS). The plane was a 737-900. I paid for the "extra leg room", but still ended up with my knees firmly in my lap. It was pretty awful! I'm not a particularly tall person, and I can't imagine what the regular economy class seats would have been like.
I've never been offered an upgrade while flying with my wife. We did run into an unexpected "downgrade" once. It was on an Air New Zealand flight from LAX to Auckland on a 747. We were supposed to have two first class seats, but we ended up with one of us having to sit in business class. We switched off between the outbound and return flights. Even business class on that plane was nice.
I was fortunate during my serious flying days (mostly the 1990s). For those of us who regularly flew a large number of miles my regular airlines - Delta, then United - automatically upgraded every flight from coach to first class on planes that offered it.