I do agree I find that so very annoying how you have to go into all your diplomatic contacts and make friends with everyone again, and then re-request your alliances. And I get especially annoyed when you've got like five alliances all expiring on one turn, lol. I wish you'd get something like in Civ 5, where when your treaty's expiring your ally will contact you on his or her turn and say "This deal strengthened out relationship, would you like to renew?" and you don't have any of those errors of resigning (also like you said, annoying troop requests!)
I still really don't like thinking about being able to cancel, even after a set number of turns. For me, those treaty limits are really important part of the terms, and when you're signing an alliance (or friendship) you need to think "Do I really want to be locked into this for 30 or so turns?", and taking that away just totally erases the point in my opinion. I really love how they are now, I just do wish you have a better way of renewing them.
I do like playing on Standard maps with seven other civilizations, and I'm often allied with five of them. But I enjoy playing that way, I love everyone being my friend and knowing I'm not going to war, so I can just do my thing. Even on larger maps when I add extra civilizations, I find security in knowing what my alliances are.