Well… I did contemplate mentioning this in my comment, but decided to keep it simpler to begin with. It
is generally more work, but a part of that is that it basically forces you to do it properly, or the results will be obviously worse; whereas the few-breakpoints approach allows you to be more lazy, and
generally has less-bad failure modes, and much more commonly in the direction of wasting absurd amounts of space rather than unusable, broken layout (since hand-crafting breakpoints and such is likely to lead you to dancing at the boundaries of what works). And these mostly-mild failures are definitely encountered more frequently.
Personally, I do custom breakpoints and content-guided layout absolutely everywhere, because I can’t bear to do worse. I haven’t worked with many people in these kinds of areas, none of those I have worked with have been like me in this, and I’ve only rarely spotted work done in this kind of way. The simple fact of the matter is that most people are pretty careless about these kinds of things, and don’t tend to do a thorough job of things. Give them a shortcut and they’ll take it, and “these are the three breakpoints we use” is a convenient shortcut. The trouble is that it’s generally good enough.
It’ll be interesting to see if any of this shifts with container queries, which necessarily work more this way. Ultimately I don’t think it’ll have a big impact, because most places don’t particularly benefit from the complexity of container queries, even though they have apparently been a strongly-desired feature. I’d be delighted to be proven wrong.
But since you push: in the end, yes, most of it is just a different way of thinking and executing. But it is also harder, and pushing to do better is fighting human nature.
I’m still intending a blog post about these and related aspects of my design implementation philosophy (among which is probably my favourite non-mainstream opinion: why I actively prefer `box-sizing: content-box` and believe it’s what you should aim to use). Most people will continue using their few fixed breakpoints, but maybe it’ll help some.