Okay, I'll try to clarify what exactly I mean by "people outside a field should be able to evaluate the quality of someone's work" - especially because, as I said regarding peer review, we generally consider that it's impossible to do so directly.

It's about the question of resource allocation. Pretty much every subfield of academia is a net consumer of resources, i.e. someone outside of that subfield is funneling resources to it. That someone - no matter if it's a university, or some foundation, or a gov't agency, or a philantrophist - needs to make a decision on how to allocate resources. And, in general, they honestly want to make a good, informed decision on which projects and researchers to support; but nonetheless they have to make a decision according to some criteria. So there's no choice of "no metric", there will always be a metric and we can only argue that it should be better. And the answer to "why anybody would willingly submit themselves to that" is that duh, you don't get a choice - you can suggest a better method to fulfil their goals of allocating resources in a way that is (also in their opinion) fair and objective; but you can't get around the fact that scientists are generally funded by nonscientists. And they need(or want) to make decisions.

They could delegate that, but that doesn't solve the question about the criteria - if they delegate that to universities, they still have to decide on how to allocate between departments; if they delegate that to scientist councils uniting all the departments in the country working on some subfield, they have to decide on how to allocate between the different organizations. So no matter what, you have to compare not only quality of similar scientists, but also of dissimilar scientists working in different (sub)fields. And delegation doesn't absolve you from responsibility, so if the money is (or looks!) wasted, then that's a failure - so when you delegate, you want to require them to use objective criteria. Which is hard - I could tell you which researchers in my subfield are doing excellent work and which are useless; but if I had to justify these decisions, to demonstrate why they're not just my bias because of politics/liking certain methods/gender/ethnicity/etc then it would actually be tricky; and I think that I'd actually reach out for these metrics. And I'm quite certain that the metrics (for the people that I have in mind) would agree with my subjective opinion; on average, the great research gets cited much more and is in higher-ranking venues; while the lousy stuff gets no citations apart from the author's only grad student.

Also, there's a lack of trust (IMHO not totally unwarranted). You could get a bunch of experts who are qualified to evaluate who gets what amount of resources, you can't rely on them actually doing so - if we take spiders as a totally random example, in general you're qualified to distinguish which spider research is good and which is useless only if you actually work on spider research, most likely in one of these teams - and the expected result is nepotism, allocating resources based on purely (intra-field) political reasons. And who'd decide on how to split resources between spider research and bird research? Do you expect the spider guys and bird guys to reach a consensus? Or would it go to whatever field the dean is in? This is a big problem even currently, and a big part of why the metrics are being gamed - but at least metrics are something that require effort to game and can't be gamed totally; if we'd do away with them, then we'd be left with absolutely arbitrary political allocation, which would be even worse.

So at the end of the day "they" need some way to transform the only reasonable source of truth - actual peer-review - to something that "non-peers" can use to judge what the the aggregate of that peer review says. That need is IMHO not negotiable, I really believe that they do actually need it - they don't want to do resource allocation totally arbitrarily, they want to do it well, they need (because of external pressures) objectivity and accountability, and currently this (journal rankings, bibliometerics, etc) is the best what we have suggested to summarize the results of that peer review.

If I had to write a law draft for a better process of allocating resources, what should be written in it?