Sure, from a protocol perspective, arranging it so nobody, even the original owner, can decrypt the communication after the fact is beneficial, except that now,

Sure, from a protocol perspective, arranging it so nobody, even the original owner, can decrypt the communication after the fact is beneficial, except that now, you end up with people downgrading such communications or storing the session secret keys, the latter probably by patching software crypto implementations where possible (since I doubt any of the major crypto implementations would agree to storing those keys even with an --i-am-a-developer-really-stop-bothering-me flag). I hope I'm wrong. I don't want to end up in a world where all enterprises actively MITM and negotiate down to non-PFS protocols to avoid this, or hack up the software stacks on their machines to circumvent this (with all the overhead that implies - echoes of "requires Internet Explorer 5" for how old some of this will probably get between updates). I just don't think that the people who make use of this functionality are going to let it go, no matter what it takes. (That's not to say it's not worth trying, to be clear - I just foresee it ending poorly for everyone except for private individuals, and possibly even them if ISPs start requiring you accept their MITM CA.)