Reason???

If what you have works, use it.

I wouldn't recommend dropping out of the three most recent macOS versions and that for various reasons, but otherwise it's up to you, your apps, and your needs and expectations.

Here, I'd initially spend more focus on getting and using Time Machine for backups, on using robust and unique passwords, and other related maintenance tasks, and on learning about and recognizing common security attacks: Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

As for newer macOS versions... Apple differentiates "upgrades" from "updates". Upgrades are larger and more disruptive and can require app updates. Updates are smaller, and are not expected to perturb apps.

I would install updates within a week or so of availability. But again, there's no pressing requirement to upgrade. You will find apps that require newer versions, and the older things get, the more likely there will be connectivity issues. The folks running macOS versions prior to High Sierra are encountering issues with secure network connectivity, and have not been receiving security updates for a while, for instance.

Catalina will be receiving security updates for another year or so, if past practices follow.

Big Sur will likely receive security updates for two more years, again per past practices.

Monterey, probably for three years.

Your MacBook Air 2017 first arrived with Sierra, can run High Sierra, Mojave, and Catalina, is now running Big Sur, and can be upgraded to Monterey.

Apple tends to support Macs for upgrades ~seven years or so, though that can vary.

From that list, iMac Pro debuted in 2017; there are no older models. MacBook Early 2015 is probably the newest that isn't supported.

A bigger hardware transition is presently underway, away from Intel processors and to Apple silicon; to M1, M1 Pro, and M1 Max processors. Your MacBook Air 2017 will be approaching a fairly typical replacement around the time that Big Sur drops off of security patches. Various folks replace their Macs every five to seven years, though some intensive users and businesses can replace a little more quickly. Though replacement with a new Mac can or will mean an upgrade, if you're still running Big Sur then.

Run Catalina. Run Big Sur. Run Monterey. Whatever.

You’ll eventually be upgrading as your applications and your connectivity ages out, or as you want or need newer apps or features. Apple tends to provide fixes for the current release, the penultimate release, and the antepenultimate macOS release. Microsoft similarly tends to supports the three most recent releases with their apps.

Whatever you pick, have backups. Have robust and unique passwords. And best not get too far back, as upgrades from antediluvian versions tends to get more involved.

Not unless you want to.

Catalina will still receive security updates for a while longer.

As with all updates, downloading them means you get the newest features and security fixes. If you have no need for them, then you don't have to download Monterey at all.

The question I pose to you is: why would you want to be one version below current? Is there anything about Monterey you don't want? Is there anything about Big Sur you wish to keep over Monterey?

Unless there's a particular reason to stay on Big Sur, you should always strive to be running the most up to date macOS version you can.

But again, its entirely up to you what macOS version you want to run.

Taxpayer wrote:

I knew I had to upgrade to Big Sur, because Apple was no longer going to maintain Catalina.... Is there a reason why I should now download Monterey?

Apple will be likely be maintaining security updates for Catalina at least until this time next year when macOS 13(?) or what ever it will be comes out. As a matter of fact, there was a Catalina security update this morning.

If you find no compelling new features in Monterey that would entice you to upgrade, there is no need to. Based on past cycles, Big Sur will be supported with security updates for another two years.

As I said, only if you want to. You never "need" to update. Its always entirely optional.

Only you can know if there is something in Monterey you want over Big Sur. It does have several new features.

For now, Catalina and Big Sur are both still getting security updates. So unless there's some feature in Monterey you want to have there is no reason or need to update if you do not want to.

Though I don't have any reason for not wanting to either. Why so reluctant to update exactly? Why did you wait so long to update to Big Sur?

Do you have some issue with updating specifically?

Taxpayer wrote:

My origingal question was should I, do I need to, because of security or maintenance issues, replace Big Sur with Monterey?

Then no. You don't absolutely need to if you don't want to. Big Sur will continue to be supported for several more years with security updates.

The benefit of Apple updates is you won't be able to install it on a Mac that cannot run it. Apple Makes sure to not allow this.

If your MacBook Air is a 2014 or earlier model it simply will not install Monterey.

If its a 2015 or later model, then it can run Monterey just fine. You can check the model in the Apple Menu->About This Mac.

That's the way upgrades work. If you want one get it; if you don't want one then don't get it.