There's no evidence that the security risk for the average consumer is worsening, rather its improving (in my opinion) because of increased attention and efforts by the major OS manufacturers (Apple included) to improve their OSs by way of things like sand-boxing and segregated permissioning systems as well as from security and privacy-concerned groups.
In my view, that is kind of a contradictory statement. In other words, if the security risk for the average consumer is not worsening, then Apple would not be taking the steps that it has been taking in recent years with sandboxing, SIP, removing the ability to install software from any developer, etc. In my view, Apple's very actions are a very loud statement that they are more than aware that the situation is growing worse by the year, and that Apple devices are being attacked more than ever before.
You say that there is no evidence, yet I see evidence everywhere. As we all know, in recent years, U.S. military websites, U.S. government websites, software giants such as Adobe, large online retailers, banks, etc., have all be hacked into.
Even if hackers concentrate on major websites such as the above, it still affects us directly. In my own life, due to no fault of my own, in recent years, my credit card was compromised twice, and had to be replaced twice. That was a major hassle which took months.
At this very moment, there are literally millions of computers around the world which have become slaves to botnets, and in most cases, the computer owners don't even realize it. Granted, most of the infected machines are probably not Macs . . . at least not yet.
My point is, there is indeed evidence of a growing threat, a lot of evidence. To believe otherwise is to live in a bubble, which I personally refuse to do. I prefer to be proactive, and safe, rather than sorry.
The thing about antivirus is that even they can't necessarily protect a user against their own actions. Some might even be a risk or vector themselves. There was a damning report about Norton earlier this year. What really helps is improved literacy and understanding of where and how security threats can penetrate your system.
The bottom line for big anti-virus companies is the profit margin. Without an atmosphere of constant fear and paranoia, which induces people to purchase their products, they make no money. Thus, I have no doubt that they purposely hype up the threats in some cases. I am really not sure how much we should trust them.
By and large the biggest risk is from an installer that you yourself grant permission to run, especially if it requests administrator privileges. Very few software packages should require such access and fortunately very few do (on the Mac side at least). If you come across such an installer you should ask questions like whether it's from a reputable company and website and what the reasons are for requiring administrator privileges.
I don't know if I would classify installers as the biggest risk -- how do we honestly gauge that? -- but I will agree that installers are indeed one common way in which hackers gain access to an untold number of machines. The fake Adobe Flash installer message is one good example.
As I have said before, we need to take personal responsibility for our actions on our own machines. We need to be careful what we install, what links we click, what emails we open, etc.
Yet at the same time, there is only so much that we can do as individual computer owners. Tragically, as we have all heard and learned in recent years, some of the biggest culprits are the online companies and institutions themselves, who do not take our security as seriously as we do. Shame on them all!
Aside from this the biggest threats are probably to your own privacy, but that's a completely separate issue.
In my view, anyone who honestly believes that true, 100% online privacy and anonymity still exists is fooling themselves, and living in a dream world. Hackers aside, and national governments aside, even your local ISP, or someone else down the chain, can access a lot of your personal data if they really want to. Packets can easily be sniffed. Most things are traceable to your NIC card, to your IP address, to your email address, to your name, or to something else. Even with encryption, a good hacker only needs enough time.