It seems like it will have a major effect, especially in separating the wheat from the chaff. The people who saw this coming and prepared for it will be fine. Agencies that haven't been paying attention will be the most affected, some may even close.
But most of all it will affect the small business owners who were doing most of their SEO on their own, using basic techniques and strategies, because frankly, they didn't need to buy AdWords or hire an SEO company or freelancer. They were getting the results they needed from the tools they had, and bootstrapping into a position to hire help.
While keywords should not be the only focus of a search strategy plan, having access to the data that tells you how a person got to your site, is helpful in determining WHY people are arriving at certain pages. It gives clues on what they may have expected to find, and helps track whether what you're doing is on target.
Think about what would happen offline if you ran a restaurant, but suddenly could not find out how people are finding out about your place. Is it word of mouth? Are your television ads working? Did you get a great review from a critic?
Without that knowledge, yes, you can still figure out what to change or adjust. Process of elimination would be one way to find out - stop doing certain things for a set period and watch what happens to your foot traffic.
But it's extremely inefficient, isn't it?
While I don't think this event is a major catastrophe, I do take note when major search publications by fellow experts that I respect advise caution and report it as major news. Search Engine Land, Search Engine Watch, and other major industry publications are watching this closely. I've worked with some of the people at the major publications and respect their work, and thus their opinions.
To your second question, I think it will, because small business owners, especially microbusiness owners will look at the cost of hiring someone to do their SEO vs paying for traffic, and the same thing will happen way back when Yahoo was king of search.
They'll pay. For most small companies it will become more cost effective to pay for traffic than to pay for help, and nearly impossible to do this themselves.
As far as your third question, ROI is proven by metrics. Many other metrics will remain to show the value of search. It still drives the most traffic. Smart search professionals already know multiple ways to show how targeted traffic impacts the generation of leads and sales.