My apps usually deal with non-technical people (thus ISO in the browser input is out of the question), and sometimes spread at least between Europe and US (so people would enter MMDDYYYY and DDMMYYYY, and keep in mind that some dates work validate for both formats, namely every 12 first days for each month). Behind the scenes everything is ISO and UTC, but everyone gets their preferred format in the frontend.
I'm not getting your Dutch example. If I'm in the US and use MMDDYYYY almost always, and want to access some Dutch site, what date format would you show me in the input form? DDMMYYYY only because the backend is hosted in Europe? You will get wrong dates (e.g. 11/1/2024 for the first day of november, your system will store 11th of january) for at least half the US visitors.
Or maybe you are talking about "dates displayed" instead of "dates input", which is a different problem?