We have been ramping this setting default down – right now it is at 3 months, it originally was 1 year. @geek frequently complained that allowing people to edit very old posts could lead to misbehavior and going back to old posts and adding in affiliate or spam links without anyone noticing.
I do agree that we want to be “safe by default” and a 3 month default edit window for your old posts is safer than 12 months or even 6 months.