Eliminating time constraints is entirely reasonable. Leaving exams early is generally an option in most standardized testing systems - though usually with some minimum time you must remain present before leaving.

Taking what is currently scheduled as a three hour exam which many students already leave after 2, and for which some have accommodations allowing them 4 hours, and just setting aside up to five hours for it for everyone, likely makes the exam a fairer test of knowledge (as opposed to a test of exam skills and pressured time management) for everyone.

Once you’ve answered all the problems, or completed an essay, additional time isn’t going to make your answers any better. So you can just get up and leave when you’re done.

I think one challenge would be preventing professors from taking advantage of the time to extend the test. I suspect the professors would generally like to extend the test to be more comprehensive, and are limited by the time limits of the test, and tests will naturally extend to fill whatever default time is allotted.