Google has more adwords advertisers today than it did 15 years ago so there are more ads taking up space.
Google also has more searchers than it did 15 years ago, so maybe it's a wash.
Google has also added local listings, images, videos, news headlines, shopping and other results to their search results. Many of these changes have resulted in less traffic flowing to the organic web results. Before you could rank #3 or #4 and still appear above the page fold. Now the #3 or #4 position can fall below the page fold.
I think that's the bigger problem. I'm not a big fan of "Universal Search" myself, but Google probably knows better than I do whether users like it. Google appears to be doing pretty well these days, so maybe the dog's-dinner layout appeals to searchers. Still, it would be interesting to see the results of eye-tracking and usage studies: How many searchers scan everything, and how many just ignore the "Universal Search" bells and whistles and focus on the traditional organic results?
Also, how much is personalization affecting impressions for a given site and a given search string? It seems logical to assume that, as Google gradually improves its personalization of organic search results, those results will become more and more targeted, and the proper response to "I rank #3 for 'red widgets'" will be "Yes, if you're talking about search results delivered from 8 a.m. to 4 p.m. to middle-aged women in Keokuk, Iowa who prefer long text with photos of cats."
[edited by: EditorialGuy at 2:27 pm (utc) on Sep 16, 2013]