I haven't taken the plunge yet on any of my Macs. I don't see a benefit and lots of downsides. All of the old, expensive 32-bit apps I use a few times a year (like Sibelius) will go away, and there's no benefit. A lot of people are having issues with all of the new security, so unless the developer has done a lot of work it's difficult to install apps or plug-ins.
I totally get "if it ain't broke, don't fix it." I've got a tendency to stay a system or two behind as long as possible for the most part. There always comes a point of no return so I've learned to avail myself of options when they are still in place.
High Sierra also blocked installation from third party vendors too. My apologies if I overstate the obvious below, it may help somebody.
First, check your vendor's site for updates. Get on their mailing list and watch for sales on upgrades too, I've gotten stupid bargains for expensive software, usually the vendor is about to go from version 3 to 4 etc. If 3 works for you it might be stupid cheap.
Open System Preferences - under the Apple menu top left of the screen.
In the General tab, click the lock lower left to unlock settings. Make sure that "Allow apps downloaded from:" is set for "App Store and identified developers."
Leave that open on the screen.
Initiate the installation. If you've installed plugins from the same company in the past you may not get any notification.
If the vendor is unrecognized you should get a question block floating above, asking for permission and an "Allow" button will appear.
Allow installation, install and restart.
That's worked for me almost always. Anything that is already installed becomes an "identified developer" to the updated System. Once in a great while, I've had to install twice to get something going.
I just made a bootable external drive (well, 2 of them) and used the installation options to install my Applications folder. Was worried about losing Adobe Photoshop Elements, I use it all the time to prepare images for items I want to sell. I also wanted a path back to my old way of working if the new way had too many obstacles. Tested, both boot and have the software I wanted available. Always put the safety nets out before jumping!!! :- D
Just remember that you won't be able to access the Desktop on the new system when you boot from the external drive. The easiest workaround is to store the work files on a drive, the external boot drive, an SD card or USB thumb, etc.
When the world updates and starts leaving your computer behind, you will see the benefits. Browsers will update and older ones will be dysfunctional, just for one example.
I felt like it was important to take action while I could still log onto Apple and be able to download my choice of High Sierra, Mojave and Catalina.
When the next update comes around they may remove at least High Sierra from availablility but my boot disks will still work if/when I need them.