"The GET method exists for a reason".
There's no legitimate reason why the GET method should exist at all and should have been deprecated a long time ago.
There is no point in implementing clientside form validation because the GET method allows you to bypass it.
In fact with the GET method you don't even have to use the form at all. The URL parameters sent along with the data are not only visible to everyone in the browser address bar, but are also stored unencrypted in the browser history, cache, and log file of the server. These problems are well documented. It only remains today for legacy reasons. Supporting old websites.
"... but you have to be clear on what you're securing and why, or it won't work.".
I have no idea what that even means.
But you are actually arguing in favour of developing and implementing unsecure systems.
I find that ridiculous.