How many others are in my position? (Q Mac)

You may be able to install Parallels Desktop on your machine, then install a newer version of macOS within that. I've done it the reverse way in order to run legacy 32 bit apps. I don't know if Parallels will let you run a newer guest OS than the machine natively supports. But might be worth exploring.

I was thinking perhaps an older version of parallels. But then again, I don't know if an older version will let you run 10.13 as a guest OS.

@TPCaldwell  I know this isn't what you want to hear, but it might help if you adjust your thinking about your decade-old Mac Pro. Yes, it cost you a very substantial $10,000 back in the day, but today, a computer that costs just over 10% as much runs circles around the old beast. Whether the computer you bought in 2010 was a $1,200 iMac or a $10,000 Mac Pro, older computers become obsolete, and the sad reality is that the higher cost of a high-end machine generally doesn't bring much more longevity. You can buy a refurbished 2010 Mac Pro for under $400 today because of it's decreased value for most uses.

The fact is you're running an operating system no longer supported by Apple. It's vulnerable to malware which Apple is no longer protecting you against. Your web browser may not work on all websites, especially with many financial institutions that require more recent, more secure web browsers. Most software developers find they have to stop supporting older versions of operating systems after some number of years because of security concerns as well as the programming tools and underlying features of the operating systems changing; newer features can't be made to work on the older operating systems (or if they can, it takes more time and work than they deem viable for the small number of people using the old operating system.)

You may be disappointed in Quicken for "abandoning" users of old operating systems, but most software companies, from Microsoft and Adobe to -- as you've found -- Intuit and Parallels, no longer support those old operating systems. So it's not like Quicken is doing something arbitrary or out-of-step with the software industry. 

Here's the latest usage of macOS stats I could find as of August 2020:

macOS version        Pct of users   Cumulative Pct
Catalina (10.15)         58%            58%
Mojave (10.14)           16%            74%
High Sierra (10.13)      13%            87%
Sierra (10.12)            5%            92%
El Capitan (10.11)        4%            96%
Yosemite (10.10)          2%            98%
Mavericks (10.9)          1%            99%
Snow Leopard and older    1%           100%

Since Quicken Mac currently only runs on El Capitan and above, there are probably fewer than 10% Of Quicken Mac users still using El Capitan and Sierra. And you can continue to use your current Quicken Mac if you wish to stay on your current computer, but you won't be able to get future upgrades and at some point won't be able to do online transactions.

With a 10 year-old computer running a 5-year-old operating system, you're running into an inevitable fork in the road: (a) keep using the old system and know that you can't get upgrades and certain online sites won't work, or (b) move to a newer computer with a new operating system and current software (some of it upgraded at a cost) which will get you through the next chunk of time (likely 5-8 years for the hardware, and probably 3 years for the operating system). Factor into the decision-making that the hard drive(s) on a 10-year-old machine are likely past their life expectancy, and other hardware -- from a power supply to a cooling fan -- is going to be increasingly likely to fail the longer you use the old machine. 

The only good news, such as it is, is that it will "only" cost about $800 for a new Mac Mini that will be far faster than your 10-year-old former top-of-the-line machine. (Although if you have old peripherals, they may or may not work with modern hardware.)

Please understand I'm not criticizing you for wanting to continue using your expensive older computer; I'm just trying to provide a reality check about the issues you're up against as you continue down that road.