I've long considered writing up a feature-request like this, making blockquoted text collapsable by default, but refrained because I worried it would open up too much opportunity for abuse.
The site has had a long history of users just blindly throwing around lists of evidences (not to mention blatant copy-paste of third-party articles) with little to no context, without the willingness (or apparently even the ability) to critically analyse them. And given that these are often posted in answers to questions that also don't demonstrate the willingness (or apparently even the ability) to critically analyse them, they rarely seem all that helpful to anyone.
Stack Exchange is geared toward focussed answers to focussed questions, and blind blanket-posts are anything but: As I see it, allowing one to hide such evidences to avoid distracting others is uncomfortably close to encouraging people to stuff their answers with evidences — which may be only tangentially relevant — just because they might be useful to someone (rather than take the effort to determine what actually is useful to the questioner and focus their post accordingly).
In almost every case where I recall actually wanting to collapse lengthy evidences, it's less because they were distracting and more because they plain weren't adding anything useful to the actual answer. In such cases, they shouldn't just be masked, they probably shouldn't be there at all.
Now I'm not saying that this is the case at all with your own linked post; it appears well-written both in its original form and with the added block-quotes (although I would've gone with inline quotes, that's just a stylistic preference). But there's the rub: Your answer seems perfectly fine in both forms. Since there is no significant improvement by having the quotes, nor a significant improvement by removing the quotes, I don't see any significant improvement to be gained by adding a new feature to collapse the quotes.
The "spoiler" idea definitely has some merit here, but given the potential for abuse I would prefer to see more concrete examples where implementing it would show a marked improvement that can't be obtained by just pruning a bunch of irrelevant text.