I like simple no-frills function-first websites, but I don't consider it a good thing when an old-style website has text running across the entire width of my 1440p monitor. It's just not pleasant to read, and given that the fix is often just two CSS rules (max-width:800px;margin:auto), I don't think it's unreasonable to ask for. You can still design your website like we're in the era of 800x600 displays, but please, take the tiny step to make it play nice with larger screens too.

I think one should use ex/em units when setting a maximum width for readability. People with poorer vision will tend to prefer a physically larger font size. Pixels can also vary in size for different displays. (I am not a web developer but I do have poor eyesight and delay getting new glasses.)