No, your assumption is not correct.
At no time will you be required to install MacOS 27 in order to continue to receive future security updates for macOS 26 Tahoe.
Your Tahoe install will continue to receive security updates as long as Apple chooses to support the OS.
If you should ever upgrade to macOS 27 Golden Gate, then you will move beyond the realm of Tahoe and any updates that become available for Tahoe will be irrelevant to your Golden Gate installation.
If you decline to upgrade from 26.7.2, then you risk the security of any updates Apple provides in any further updates to Tahoe, and you forgo any improvements that Apple releases in macOS 27 Golden Gate and the OSs beyond that.
If your Mac is capable of running a newer OS, but you decline to update or upgrade, in the long run your Mac will age out of productive use long before it is necessary. To get the longest life from your Mac, keep the OS updated and upgraded as long as Apple makes OS versions that it is capable of running.
You don't necessarily have to use every AI feature that a given OS provides, just as you don't have to use every app that Apple includes in the OS installation.