Don't have any hard facts, but in thinking about it seems that way to me. Best answer making a case for, or against this is select.
I don't believe so. I consult on both PPC and SEO, and by far I get far more interest in SEO. However, it's still a struggle for many to dedicate time and resources to SEO since it's more difficult to show ROI.
I've gotten good at showing how much money it would cost to buy the same traffic via PPC - it's a very high recurring cost, versus a much lower and infrequent cost for the same traffic via SEO.
Both PPC and SEO are part of an overall search marketing effort. PPC provides more control, is easier to track and measure, but is expensive. SEO provides less control, is more difficult to track and measure, but is very cheap in the long run.
The Google Tax continues to go up and that's when most companies seem to wake up and understand that they can't rely on PPC alone.
After late 90s bubble, only now people are taking web as serious business and making money from it. SEO and ads are now taking a solid place.
With more and more small business getting into web market everyday, I following makes sense for me.
Years ago, only large/medium business dive seriously into internet. And they had/have enough resources to invest in SEO, usability, and so on. Small business arriving now, want low cost solutions, with quick return.
SEO demands much more resources, but last longer. SEM, provides a quick return for little investiments, making investors happy.
I have a friend who sells professional kites and have a lousy site from SEO and design point of view. He spends around US$ 100 per week, and makes 15-20 sells with this, what makes his investiment worth a lot (he pay his bills, health insurance, bla bla bla common safe life). This is why he doesn't care about his site, SEO and all that stuff. Because he already have what he needs from using SEM.
I don't actually see SEO loosing terrain for SEM. They are both related but different things. I see a huge growth rate for SEM, in comparison to SEO, but both are growing in market.