I think what will change is the amount people will be willing to pay for the ads. I believe this will mostly decrease as time marches on. The good news for Google is that the number of advertisers will far exceed any losses in revenue from lower click through rates.

I have first hand experience with this with two clients:

Client-A is spending thousands of dollars per month with Google and happy to do so. When he signs up a new customer, he could see revenue from this customer for years. So the return on investment is high.

Client-B is more the norm. This client is selling a widget for $29.95 or $99.95. There is virtually no chance for a follow up sale. I've yet to see this type of client successfully use Google paid advertising successfully.

Client-A is the exception, not the rule. Client-B is the typical guy, and until Google can get their advertising rates down, these clients probably will not be using the engines.

Just one man's take on it.