Not me. I've never understood that. I'm still using a 10-year old 4G phone. People like you describe act as if they actually have money trees, throwing away an over-$1000 piece of equipment every 2-3 years just because there's an even more expensive version out!
Because that's how the World Wide Web was designed to work, that's why! The original idea behind HTML was that, no matter how many fancy bells and whistles were added later on, a Web page should still look the same to folks using a browser that didn't support the new bells and whistles. The page may be slow and look like one of those ugly pages from the '80's, but it's still supposed to work. (And for the most part, the WWW really did work that way for its first couple of decades.) I know we got away from that ideal long ago, but I still think it's an ideal worth striving for, rather than Discourse (or whoever) shutting you out of their sites completely because your browser/OS doesn't support all the HTML features they think they might want to use someday.
Someone once told me there's a difference between dealing with organic change like the shifting seasons, and the change forced on you by someone who is whipping you forward like a drover, toward a destination of their choosing, not yours.
You misunderstand the situation. We don't get to choose whether to use "old school" or "new school" methods! If the Web designer used the "old school" method of sniffing the UA (e.g., chase.com), then we have to use the corresponding "old school" method of spoofing the UA just to get in! Of course, we often have to use "new school" methods as well, like those built into a Web browser like R3dfox or Supermium, or the site will likely not work well (see above) but that doesn't mean we can ignore UA spoofing just because it's considered "old school."