They'll just think "oh god, I have viruses!" and call whoever is the family's "computer whiz". The very concept of "this system is not secure" is something that ordinary user does not understand.
People understand if a system is insecure, and now apparently all non-Yosemite Macs are insecure. Folks will grok that. Whether or not they find out about it, well that's up to you and I, and everyone else on this website.
I didn't mean it in a cynical way. From my observation, "it has viruses" is the best model that is self-consistent and yet doesn't require investing a lot of time in comprehending the workings of a computer. Non-tech people reach it naturally.
I agree it's up to us to proactively tell our parents, friends and colleagues who may be vulnerable about this problem.
I have to go with the idea that MOST people have no idea about security with their computers. Also most Apple people still say no bugs, no crashes and no viruses. Even when I point out every time something crashes on their Mac.
After getting more and more tired of Apple's generally obnoxious behavior over the last few years I'm wondering which straw will be the last for me before doing exactly that. I'm pretty seriously considering wiping OSX off my MBP and running some variety of Linux on it at this point, though regrettably I might still be forced to consider them for future hardware purchases on account of the fact that I don't think I've ever encountered a non-Apple laptop that seemed worthwhile (in terms of general construction and build quality).
In danger of being accused of flogging deceased equines I can but point at the T42p ThinkPad which I'm typing this message on for an example of a device which fits that premise: solid hardware and a generally well though-out design. It is 11 years old, but sports a screen which comes remarkably close to the more recent Apple offerings. It also runs the latest software flawlessly, as long as that latest software is a recent Linux distribution. It has a keyboard which Apple-adepts can only dream about. Ports aplenty, two batteries or drives or whatnots, no problem, ~8 hours on a single battery charge when using the oddly shaped 'extended' battery.
An additional bonus is that you can get these things for free if you know where to look.
Of course, being 11 years old it also has its drawbacks. It won't fit more than 2GB of RAM. It is based around a single-core Pentium M which generally performs fine but shows its age mostly when meeting Javascript-hobbled web-related things.
BUT... and this is one of the main reasons why I use older hardware... if you develop software on this machine, and it works fine on that machine, it'll run circles around the stuff your neighbour made on his latest FlitzBang Fruitmachine. It'll but cause a blip on the CPU meter where his (or her, your choice) result maxes it out. It'll use memory like it was rationed, not like it was on sale.
Of course you can not develop software for the dark side on this older hardware. A small loss, in my opinion.
I'm working on a year-long government project now to migrate a small bunch of websites from Windows Server 2003 to 2008 R2 (because extended support for 2003 ends this summer). Yes, we're upgrading to a 6 year old operating system.
The majority of their user base doesn't know and wouldn't care about this type of thing even if they did.