Advertising was a mature market when Google started. Media was a mature market when Facebook started. (And, to make a slightly different point, people believed computers were a mature market when Apple started.)
Is the question the expansion in adtech companies? If that is so, I think a huge part has been the Facebook traffic dumps to publishers like news sites. Basically the available display inventory something like doubled. That isn't sustainable, Facebook fully intends to recapture that external traffic back in to their platform. The drop is already visible to some publishers. Many companies certainly are turning a blind eye to fraud so as to hide the bad news from their clients and investors.
At some point the shift from offline to digital ends and the growth moves more in line with GDP. That will bring down valuations. Google and Facebook probably want to integrate the ad model with every human transaction. One could imagine getting in a Google car and saying "pizza" and be driven to the highest bidding pizza shop.
Certainly ad costs get baked in to some products in a big way: Coca-Cola, Geico, hotel rooms, for profit universities, and so on. Sometimes the costs are so much the profit center moves to another area, such as investing the float from insurance or maybe just straight up stealing money out of a person's bank account (pay day loan lead aggregators selling bank info to criminals, has happened many times, see FTC.)
As a whole, I don't know how this plays out. If the ad competition is brutal enough the costs will be driven up to the point where the parties not spending money on advertising are just far better deals. If it isn't the prices running up, the cost could be offloaded somewhere else like cutting maintenance at a hotel. Eventually the advertiser's reputation tanks.
These are marketplaces. On a micro level there are many perverse distortions because of some condition at a given time. Over time they should re-balance.
Some things like Coca-Cola, which has probably physically harmed and killed people on the scale of cigarettes, just last a really long time. Coca-Cola has a truly perverse combination of trademark law enforcement, addiction (caffeine+sugar), general scientific ignorance by consumers, and a long list of government subsidized cost advantages. All of these strengths save trademark law are being chipped away at right now. Which, at some point it flips and they have the McDonalds problem (toxic brand.)
You're right that this is part of the normal cycle of capitalism, with all the creative destruction that entails. It's quite normal for a new, growing industry to support a whole forest of new businesses, and then as the industry matures some of those will die off. But I think there's one extraordinary thing here: the advertising industry is growing like a new market, the kind of market you see when the customer base is growing by 100% every year, like the early days of the Internet. But that growth simply isn't there; advertising is by most measures a mature market. So what's fueling all this expansion?