But temporarily switching to a backup machine can be done with the click of a mouse, if you keep your machine backed up to an external drive using Carbon Copy Cloner, using the bootable clone option.
Ok, but when you move machines, it's never exactly the same. Here's what I'm getting at:
When you migrate, most of it just works, but some of your licenses stop working. Apple Music has to be authorized on your new computer, but then you realize you're out of authorizations so you have to deauthorize all of your other devices and now you have to reauth on all of those.
Some of your keychain items aren't recognized automatically by your programs anymore. They're still there, but for some reason your new machine won't access some passwords automatically so you have to manually fetch them. In doing so, you make a mistake and now you have competing passwords and you're not sure which one to use.
There's a program that you don't use very often, but when you do it's very time sensitive. It won't run because the license was bound to your other computer. You need to jump through hoops to get it authorized your new computer, but you bought this program 4 years ago and can't find the license. Now you spend 2 hours trying to find that license code somewhere.
Then once you get snared in loops like this, you then start getting confused and make stupid decisions like deleting the wrong ssh keys to your developer projects in progress and now you can't deploy for a client that needed something last week. You then have to re-authenticate with identity providers, some of whom are actual people who do it wrong on the first try or reply back a week later.
This is what I mean. The issues you encounter are minor on their own, but if you have a complex enough of a setup they will sometimes snowball and become bigger than a sum of their parts.
That's why I try to keep my upgrades to once every 4 years or more.