Not trying to be argumentative, but I’m genuinely curious; how can any other quality be objectively measured?
You mean like gloss, chemical resistance or durability? Easily. There are gloss meters, and you can drop different stuff on coated and non-coated surfaces, and check how easily they can be removed. And durability you can check by monitoring the change in these properties over time.
If we assume that the presence of a freshly laid down wax or sealant comes with a certain water behavior - say Collinite 845s aggressive water beading, and we see it suddenly stop beading after a week, would it not be reasonable to think the protection failed somehow?
No, because as I explained, water behavior depends heavily on the surface you're applying to coating to. If it's uneven (meaning it's full of major or minor scratches), a thin sealant like Reload won't be able to compensate for that, which in turn will result in not so great water behavior. However, it might still enhance the gloss or the chemical resistance of the surface better, and last a longer time, than a different sealant or coating, that seems to provide better beading, but is not as much chemically resistant, or will degrade faster over time, than said product.
Or if we had applied a fresh layer of 845, and over the weekly maintenance washes notice degradation of the water behavior - would it not also be logical to think the protection is wearing down?
No, because water behavior and chemical/physical protection are affected by very different things. And the degradation or even initial low performance of one doesn't necessarily mean degradation or low performance in the other area. If anything, gloss is what could be relevant to water behavior (because they both depend on the surface being perfectly even, at least on the micron level) - but even that is just partially true (because differences at the nano level might result in very different water behavior) .
I don’t disagree that some LSPs protect more robustly from big/bird poop etching, but I just don’t see how it can be objectively argued that protection is still on the paint if the behavior has changed so drastically.
I didn't argue for the protection being definitely on the paint even if beading is not present. What I said was, that even if beading is not present, there still might be protection there. Which are very different things.
I also feel like the water behavior of an LSP is one of the biggest practical benefits of any LSP. Isn’t that the reason coatings are so popular? Because of the longevity and self cleaning properties?
Again: longevity is not related to water behavior or self-cleaning properties. There are protections that provide virtually no visible "edge" in these areas over regular paint (or might even perform worse), yet provide immense protection against both chemical and physical impacts, and even provide more shine. Like clear bras.
I know if I were to put a sealant or wax on and the water suddenly stopped beading, and I was told by the manufacturer to trust that it was still there, id simply move on from that company
Well, that's your choice, and you're obviously free or judge a product any way you want. But really, just because beading stops, doesn't mean anything about the sealant being there. For ex. if you've pollen contamination settled on the surface (even if just one hour or one day after application), you'll experience terrible beading, even though the sealant will be obviously fully in tact on the surface - just below the contamination, that will ultimately affect water behavior.
because I’d likely not enjoy washing and drying a car that behaves like an unprotected one (no matter if the LSP is still there or not.)
Again, this is your prerogative, and it might even be a valid point for evaluation or preference amongst different sealants and products. Meaning, it's just as valid to buy or like a product for the water behavior it provides, than for liking it for the chemical or physical resistance, shine, or any other property it might provide. However, it makes no point trying to make claims about one property of a product, while assessing or observing actually a different property of it - ie. claim that it's not durable or doesn't provide protection, just because it doesn't bead water the way you like it.
That was really the only point I was making. And everyone is free to use whatever product they like anyway. I'm not here to convince anyone to use for ex. Reload if they are not convinced by it. I'm just merely point out practical flaws in the conclusions some people claim to have arrived at by observing this or that.