alt Hacker NewsI don't know about "most". For various reasons, I use a 2-year-old browser on a daily basis (alongside an up-to-date browser), and I routinely run into websites that are completely broken on the 2-year-old browser. Unrelated to outdatedness, I recently ran into a local government website that e-mailed me my password in plaintext upon account creation. I have no way of accurately quantifying whether "most" web developers fall into the competent or incompetent bucket, but regardless of which there are more of, there are a significant enough number of incompetent ones.
I think a very common browserlist target is "last 2 version, not dead, > 0.2%". So if you have a 2-year old browser you are probably dozens of versions behind and are very likely in that 2% of users which developers simply ignore.