Just to elaborate a bit, it's pretty easy to keep three copies of stuff you care about. First, you have the working copy, on the machine itself. A big backup disk is pretty straightforward. I use time machine, which isn't super reliable, but it's very very easy.
Now, when it comes down to it, do you really need to backup the OS? or your installed software? if the machine and the backups fail, you're going to be reinstalling anyway (probably) So, for the third copy i rely on 3rd parties. Different people have different needs, you might want to do something fancy in house.
It pretty much boils down to finding a service for your stuff. I have a couple of private github repos. Photos on iCloud and whatever Alphabet is calling Picasa these days. 20 gigs of music to Amazon or Alphabet (or both). Administrative stuff, like taxes, i just email to myself. It's probably smarter to keep that in dropbox or something along those lines.
The key point is, there are the things you make or capture that are irreplaceable, save those lots of places. There's a bunch of other crap on your computer to make it be useful. That stuff is trivial to reinstall. Well, ok, it might cost you a day or two to redownload and reconfigure emacs just so - but with a little planning you can put that config in git, so it's easy to restore or set up on a new machine.
It's almost better to think in terms of, if i had to upgrade tomorrow, what would i need to copy over? that's the stuff to be really fussy about.