Security concerns are mainly overblown on Macs. They don't really get viruses. Malware is pretty easy to avoid. Just use your head and keep moving forward on HS. Hardware will likely begin to fail in the next couple of years anyway.
I kind of agree with the overblown concerns with viruses and malware with Mac OS.
As long as you are wise with what websites that you go and visit, especially what apps you download etc., generally you are fine. I don't like Crome, for it is a resource hog (though it is getting better) and who knows what information Goggle is harvesting... Regardless, there are options that will continue for some time for web browsers that will support High Sierra, so if you are so concerned about it, find a supported browser and you should be fine.
This concept of leaving to go to another OS because a few year old OS is not receiving security updates is very exaggerated in my opinion. Many bloggers on these forums are from those "enthusiasts" who like to do things that are not the norm for most users and is why the concern for security etc. and will inform people to update or go to Linux (understandable). My concept is "You play...you pay..."

These are wise and good options (if you play), but for normal
typical users, there is no need or really any concerns.
If you are a light user (and don't go to obscure websites, hack or pirate software etc.) then you probably will not experience probably any viruses or malware. Currently with internet technology viruses and malware cannot just "jump" on your system with Mac OS UNLESS you invite it through downloading or clicking pop up boxes etc. that you are not familiar with.
All of that said, unless you are an adventurist or do non-typical things with you Mac, using older OS's are generally ok.
High Sierra is a decent OS, so if it works for you then there is not reason to even update or change. Just get a supported current browser if you are concerned. macOS Mojave is a good option if your system can handle it. Probably the best in recent years overall for most "normal" users. I still use Snow Leopard for some programs and IF I need internet, I use Arctic Fox browser that is the only one that I have found that still supports the OS. No issues, but I know what to avoid...
