I think you missed this page: https://10yearphone.com/sign-the-letter/
The whole page is bullshit to argue for Right to Repair, et. al. Not that Right to Repair isn't important, but being weaselly on your web page is unlikely to gain supporters. Go ahead and click that "Get the Phone" button. Haha! We really want you to sign a letter that no one will read!
And that's completely setting aside how unworkable this would be in reality (think about things like hardware support for encryption standards and codecs in use in 2032).
Even if the OS gets security patches for 10 years, does anyone think app developers are going to continue improving and supporting their apps on this thing?