A solution would be one which prevents the sending of the spam in the first place, or an auto-rejection mechanism that was 100% passive and 100% accurate.
Spam is merely a problem that is manageable with some ultimately clunky tools.
How long would your email be usable if you turned off your spam filters?
Engine aire filters are also preventing the INGRESS of foreign contaminants. Most spam filters are much closer to the receiver than the sender. So the spam email volume still moves through many servers, routers and networks before it is finally filtered out and stopped.
I don't think your analogy holds up at all.
One is managing _your_ inbox. Yes, the tools are there, and if you're using any largish email service provider, you've probably got pretty effective filtering going on.
Try getting on the other side of it, though, and start _sending_ email from a new domain. The same mechanisms which "solve" the inbound problem create a _huge_ issue for outbound. Especially if, say, your business is somehow reliant on being able to reach out and communicate a message to people (I'm not even talking about marketing -- think of status notifications, account mailings, etc., etc.).
I'm a huge believer in email, don't get me wrong. I dislike most of the tools that have come along to "replace" it to some degree or another (how fast would SMS disappear in a mushroom cloud if spammers were to hit it like mail?). But it's gotten really, really creaky.
If you use any of the major mail providers (gmail, hotmail, yahoo), you should not see 75% spam in your inbox. It's worth noting that email that's "not worth looking at" is not necessarily the same as spam. The group you mentioned includes both real spam (phishing, pills, etc) and gray mail (newsletter, notifications, etc). If most of your inbox is gray mail that you're not interested in maybe it's worth setting up some filter rules or consider unsubscribing to those mailing lists.
Gray mail is generally a difficult problem for spam filters to solve since a critical email to one person might be junk to another. There are evolving ways to improve this filter but it usually requires action on the user, such as clicking the junk button or deleting them without reading.
Spam doesn't break into my house, pour yellow-brown crud all over the background, smash the walls, and take all my stuff.
It's shown me a few things that can't be unseen, though console-mode clients have distinct advantages sometimes.
The implication that there even exists an actually reliable way of communication I don't think is true either, that's why email is still so widely used.
These services are great as long as it's part of a small, insular community of like-minded people. Once the criminal element sees it for the opportunity it is, it's over.
There's an obvious analogy here to email and spammers.
I suppose it could still work if the vetting process is better, but there will always be those who mask their identity and "sneak in". The cost of this to a criminal is probably minimal while the potential rewards are great. Perhaps the key is to create smaller "circles" of trust within the wider community.