I like integrated, but most of the bikes I ride are still on the down tube...with only 6, 7, or 8 gears. For me, and my type of riding...shifting on the down tube is no problem at all. I do have a very modern CF road bike with integrated...but it's set up for triathlon and I only use it in races and race prep...so it doesn't get ridden much. I like my older road bikes...steel, ChroMo, aluminum. I've rebuilt/restored/maintained them myself. I would switch them out to integrated...but that involves so much more than just switching the brake levers. You also have to consider increasing the number of gears to match modern integrated shifters. That means new cassettes, and chains. If the wheel isn't dished to accommodate the new cassette then it has to be replaced (or re-dished). And then if the frame/dropouts aren't wide enough to accommodate the wider wheel then that has to be dealt...widened if possible. Likely have to get a new rear derailleur too.
Dan