"1) Want to be increase the chance it gets read, as opposed to thrown away."
I get all my monthly invoices per email, I throw out everything I get via snailmail, but I don't live in the US, so YMMV.
"2) Want to send something physical, such as a key."
Really? Envelopes containing such hard things get shredded regularly in the scanning equipment and lost.
Also, it's not secure, nobody sane does that.
" This also includes any letter you think your great grandchildren might want to read some day."
Only you think that, your grandchildren (great or not) don't and won't, unless it's your will and even that will get read to them by somebody else.
"3) Want to send something that you don't want copied/replied/forwarded/subpoenaed in a law suit (A lot more important than you might think)."
Cut-out newspaper words with ransom demands perhaps, for the rest you use a courier service.
"4) You don't know the recipient's email address."
And the living address you get from where? The landline phonebook? You really DO have great grandchildren.