> The reason they prevent downgrades are to reduce support costs, to ensure broad compatibility, and because of security flaws in older versions
That's a reason, not the reason. I'm sure you can think of many other reasons a company would want to prevent downgrades that they won't tell you because it's "user hostile".
For example, say an exploit is found in the Nintendo Switch that allows arbitrary code execution. People use that to load emulators, homebrew games, pirated games, you name it into their Switch console. Nintendo does not like this, so they force an upgrade on everyone, lock it from being downgraded, and now anyone who upgraded can no longer do that. They'll chalk it up to security of course, because what company will flat out say they don't want their users to have the ability to run unsigned code on "their" device?
For the specific example of Nintendo, the codephrase for their exploit-closing patches is 'Increasing Overall System Stability'[0] which has spawned a beloved meme.
That's a reason, not the reason. I'm sure you can think of many other reasons a company would want to prevent downgrades that they won't tell you because it's "user hostile".
For example, say an exploit is found in the Nintendo Switch that allows arbitrary code execution. People use that to load emulators, homebrew games, pirated games, you name it into their Switch console. Nintendo does not like this, so they force an upgrade on everyone, lock it from being downgraded, and now anyone who upgraded can no longer do that. They'll chalk it up to security of course, because what company will flat out say they don't want their users to have the ability to run unsigned code on "their" device?