@jdrichardson 

if I do not upgrade and keep using QB21 what is the downside? I do not want to compromise my accounts by having outdated software.

If you don't need to run any add-ons (e.g QB Payroll, QB Payment),  you should stay with your QB Desktop 2021. If you need to run the bank feed feature, you may have 2 options. There is nothing to worry about if you are using an outdated version of QB Desktop.

what is the reason that QB/Intuit are not selling the upgraded versions to U.S. markets? 

What do you mean with the "upgraded" version? You can still purchase QB Desktop 2024 US version. The thing is, you won't easily get Pro Plus or Premier Plus because their real intention is to force you to upgrade to QB Desktop Enterprise or switch to QB Online.