Even taking into consideration all that you've said, some high-quality sound clips recorded through decent gear will at least give you some what of an idea of what the pickups will sound like , either mic'd through a cabinet or direct through a microphone preamp or direct box. Of course all these different factors that you've listed will affect the tone but I don't think it affects it so much that you can't get somewhat of an idea what the pickups will sound like in real life. Especially if the clips are decent quality it sure as heck is better than nothing at all! Case in point, I've listened to lots of clips of Seymour Duncan spb1 pickups recorded through some kind of direct input and posted on Soundcloud or YouTube. When I listened to those clips through my headphones it sounds an awful lot like my bass with Seymour Duncan spb1 pickups direct into a mixing console and into my headphones, in fact so much so that I would say that many of the clips I've listened to provide an accurate representation of those pickups through that recording/listening context. Of course as you make the point, nothing beats hearing pickups in Your Bass, through your gear to know what they will really sound like, but I think that whoever made the post lamenting manufacturers lack of sound clips does have a valid point.
While I agree with you, I have myself encountered a lof of people, say in the range of 50/50 that have been very disappointed when judging
only by the soundclips, of which there have been several. They think they can make their Ric sound like a MusicMan and vice versa only if you'd change pickups, and so on. You MAY hear the frequency range, and lack of hum or buzz, but that's about it. But I know of as many that has been very disappointed when comparing IRL what they got to the soundclips. I think for example Q-tuners soundclips where all displaying some sort of high frequency show off, and just played harmonics on the bass string. Not ever "real" bass with low notes. And still, you can't ever, and won't ever hear how strong or weak any pickup are in terms of output. What they DON'T do are soundclips providing relative comparison within their own pickups. I mean at the same soundclip, on the same bass they use switching to compare one model against another. Then at least you could hear SOME relative difference between the two. I e they must be placed at the same spot, so it's different to do live and IRL, but with heavy editing it would be more easier to predict some difference in outputs and so on. And some differences in output volume when switched, tapped coils, phase reversed. Some have huge drops in volume and some not.
Still, I say it's 50/50 of the people I've met who have been equally satisfied and equally disappointed, because they actively searched for pickup makers that had soundclips on their sites, since you can't really buy them, with return policy. Unless it's EMG with solderless, it's quite some "damage" you do to any pickup with soldering, cutting wires and so on, so they're not definitely new when you return them back, and not even "may show some sign of wear".
Me myself have this "spice shelf" stance of pickups. You go into a greocry store, and there's this shelf of 300-400 different things. What are they all good for? What should I use it for? Is it my TASTE? You can't listen yourself to a taste, you can just play (taste) yourself to see if the taste is up your alley or not. Would be quite something if you had some internet vendor who sold you spice, and had "taste samples" for you to download, and rely on another persons judgements.
I'll say still, it's would be something similar to do demo soundclips of any stereo hi-fi speakers out there, in like "Hey listen to the bass of these speakers, how transparent they are!!!" and all you hear through are your headphones, or laptop speakers. BTW someone who's actively seeking high end boutiqe pickups like rare-earth or even Seyomour Duncanns with Zephyr silver wire and judges them by soundclips on the net (at all, not even only), has sort of revealed themselves of not being that person who should plunge into those high end pickups in the first place. I tell you all of the sounclips on YT on these Zephyr pickups (mostly guitar) have rendered comments like "what's for, like 300$ each?!?! Can' thear it" while the early adopters say in the same comments "Wow, they're well worth it, you should NOT judge by these clips, you won't hear the true nature of them, until tried IRL" and so on, and on and on it goes. There's a law of diminishing returns even there, when producing as high quality soundclips as you can, and out them out there. You still can't and won't hear how LOUD anything are with those pickups, or anything else really. Say, acoustic guitars and the like.