I just wanted to add this little piece of math to your discussion:

If a single spam domain can generate $100 per day, every day that it remains active and listed in the SERPs adds another $100. If it stays for 30 days, it means $3000 (all example numbers, don’t ask me how spam makes this money…). If the costs to buy the domain, put up the pages, and work the links to get it into the SERPs took 1 man day (and it probably takes less) add up to, say $200 (value) then you are looking at a spammer making $2800 per day. (assuming he or she gets 1 new spam domain per day into the SERPs) Not too shabby, right?

Now, if you filter the spam out quickly (say less than a week) he goes from $2800 a day to $500 per day. Filter it out even faster, and the profit drops even further.

More importantly, if Google was active in scanning the the top SERPs for spam, you would see that number drop even further, as the lifespan of the most profitable SERP spams would be very small indeed.

If Google worked a little harder at fighting spam QUICKLY as opposed to strictly by algo changes, they would kill off the big profit motive from spammers, and as a result, there would be less spam to remove because it would be less profitable to start with.

Think of it as the “broken windows” theory of spam. :)