Yes, making GUIs responsive isn't as simple as just "don't run stuff on the UI thread". There are good reasons to run stuff there even if you're going to hang up the app for a brief period, namely, the user won't see partial/incorrect updates like non-syntax highlighted text or incorrectly clipped shapes, and - especially important - it means you can't end up with invalid GUI states like the user pressing a button that does X and then immediately pressing another button that does the opposite of X, where you should have disabled the other button but didn't get to it in time. Web apps have this sort of problem if they aren't using enough JS, and it can cause all kinds of weird bugs and errors.

The reality is that moving things off the UI thread is always a calculated engineering decision. It can be worth it, but it complicates the code and should only be done if there's a realistic chance of the user noticing.