I think this state of affairs is nuts. With the exception of Firefox, because web browsers have an inordinate number of security issues to contend with.
Issues like this exist at all layers of the stack, so anything touching the internet needs regular security patches.
edit: a word
According to the proposal[0], leaf certificates are prohibited from being signed with a validity window of more than 397 days by a CA/B[1] compliant Certificate authority. This is very VERY different from the cert not being valid. It means that a CA could absolutely make you a certificate that violated these rules. If a CA signed a certificate with a longer window, they would risk having their root CA removed from the CA/B trust store which would make their root certificate pretty much worthless.
To validate this, you can look at the CA certificates that Google has[2] that are set to expire in 2036 (scroll down to "Download CA certificates" and expand the "Root CAs" section) several of which have been issued since that CA/B governance change.
As of right now, as far as I know, Chrome will continue to trust certificates that are signed with a larger window. I've not heard anything about browsers enforcing validity windows or anything like that, but would be delighted to find out the ways that I'm wrong if you can point me to a link.
Further, your home made root certificate will almost certainly not be accepted by CA/B into their trust store (and it sounds like you wouldn't want that) which means you're not bound by their governance. Feel free to issue yourself a certificate that lasts 1000 years and certifies that you're made out of marshmallows or whatever you want. As long as you install the public part of the CA into your devices it'll work great and your phone/laptop/whatever will be 100% sure you're made out of puffed sugar.
I guess I have to disclose that I'm an xoogler who worked on certificate issuance infrastructure and that this is my opinion, that my opinons are bad and I should feel bad :zoidberg:.
[0] https://github.com/cabforum/servercert/pull/138/commits/2b06... [1] https://en.wikipedia.org/wiki/CA/Browser_Forum [2] https://pki.goog/repository/