We don't. Professional baseball coaches and scouts do. It should be much more nuanced than just an arbitrary number of PAs, IMO.
I really don't want to turn this into the same discussion as we've already done, but it's absolutely the core of this thread.
You prove what you can do (that you're 'ready') inside the crucible of competition. I agree that the people that matter might be looking at and assessing things different than joe-public, but we all see the only crucible of performance that exists. There's no competition we don't see. You can't prove it in the weight room or in actions on the back field. That stuff matters, but it's only in the pressure of actual competition that you merit all of the other work.
500 PAs (for me) is the volume, more than SSS, that you need to perform under. You can't have a hot month at AA and everyone declare you ready. That's just not the way baseball has worked anywhere, ever. Performance over that 500 PAs is an assessment period that 'the professionals' assess under and they may be looking at or considering more than just rate stats the public consumes. That's fine and I agree with that, but you have to have some non-SSS period of performance under which to perform.
Zero idea why so many have a hard-on to see Holliday in the Majors. There's zero expectation that he's some hammer at 20 years old with one season of professional experience under his belt. Being able to stand out there doesn't mean he's performing meaningfully and there's absolutely a consequence.
Nobody is making the same argument for Coby Mayo. Or Norby. Or Hjerstad. Or Cowser. Or Stowers. Or really even Ortiz.
For some reason, Holliday (at 20) is the only guy that has to make it now.
I'm not trading a developmental year for a more developed year and that has zero to do with Service manipulation.