I have never needed antivirus on win7 following safe computing practices with a modern browser.
The thing is, looking forward, how long will modern browser support remain?
Besides that, security, meh nobody has ever been able to state a single thing that my win7 boxes were vulnerable to in my very typical HOME uses of them, and so so many years later after being online the whole time, never exploited, so I call security a nonsense nothingburger. If you are behind a router, and firewall is enabled, security is about entry points and that's your applications.
If you want to give up the compatibility of Windows and spend a lot of time learning 'nix, that's a hobby that you can choose. People running 'nix often pat themselves on the back that they could eventually get most of the functionality they had in windows, if they settle for constant compromises and learn over and over, every new software they're stuck with. Seems like gluttons for punishment to me, like no life, this is their hobby and it's good to have hobbies but mine isn't going to ruin my productivity at things I do daily. I'm a contractor and work online for several clients and 'nix for that? Heck no, I need to get things done NOW, not after jumping through hoops.
My point is, 'nix is a choice. It's not a bad choice if you have the time to invest in it, but it doesn't really have much to do with "I have older hardware". That is the least of the concerns because you're not talking about hardware so old that you're limited to something like 1GB system memory... If you want to go 'nix, it's every bit as applicable to the newest hardware support available. Do it or don't, but don't think it's relevant to having a little bit aged system.
I ran a self built dial up 'nix router 30 years ago. The light footprint was great, it taught me that 'nix is great for single purpose systems. I revisited it every now and then, and it's still great for limited purpose systems. Many more apps now, much better hardware support. I don't want much better. I want the target OS even if it means running older apps that don't have security issues because don't need internet access to run.
I don't want 'nix to look like Win7. I want win7 to look like Win2K.

That was when the GUI and settings interface was no-nonsense instead of color gradient impediments to what icons were supposed to be, and extra wasted space and clicks to get anything done. [/rant]