We're talking about the global spam problem, not your localised end user spam problem. Even if gmails spam filter was perfect and stopped all spam (impossible), they'd still have a problem because they'd be using vast amounts of human and computing resources to do it.
Maybe at first, but I think the GP's idea is more generally to make sending spam not a viable way of making money. If end users never see spam and spam profits drop from 0.2% of email addresses or whatever they're at now to 0.000000000002% then other methods to make money will be used.
I already specified that perfection is impossible. Even if every email is routed via a human filter instead of a computer filter, there would still be mistakes.
If a spammer can't get a message through, they will keep tweaking it until they succeed.
If even 999 out of 1000 spams are blocked, they'll still keep firing.
