I would like to strike a better balance between charging by the hour and complete fix price, especially where the work is hardly predictable up front. The problem here is a mix of trust, respect and discipline. If both parties share these values and an hour really represents useful AND needed output than the actual time spent would always be larger than the billed amount. We do have such understanding with some clients and consultants/ contractors. This works well if the work is on-off, shorter projects with people that have a long-term professional relationship. Of course, such an approach will never go down with people who have a stubborn accounting mindset.
Within the tech industry, we rely on people to think things through well. Because like with other engineered systems actually changing things later has a real cost, even though it's all an artificial, massless construct and even though we do have AI to do some of the grunt work. The problem is, we are building understanding, predictability and bigger changes tend to make some assumptions obsolete. Sometimes you don't even know which exactly, unless you have precisely engineered the change - costing thinking time that is mostly invisible, e.g. people only write down the result of the thinking or the gist of the straightened path to that result if you are lucky. Almost nobody writes down the paths not taken and the reasoning for those decisions along the way. All of this is the proof of work that's missing or that's hard to verify, if it was created honestly and not inflated artificially.
So yes, measuring work, efficiency of spending time doing work and agreeing on compensation are the hard parts, especially if we cut trust out of the equation.