And.... game over. The end has come.
As for the recurring threads, here's the truth- relationships take work. These two poor kids have only had three months. That's not enough time. So this was doomed from the start.
And that was before adding in other things. Primarily, their parents. Nobody Special has hit the nail on the head about their parents, but it's also an opener to a more general point about their personalities. Aggie's biggest problem and greatest strength is her idealism. It makes her alternatively very driven and very naive; were this a fantasy story, Aggie would be the paladin character. If set in space, she'd be Doc McCoy. She's devoted, she's committed, she's unflinching, but her fascination with and eternal drive towards her perfect ideals makes her unable to see in gray; it is white, black, liberal, conservative, with her. As merely one example, she overreacted to Penny's accidental refusal of Duane by accusing her of being worse than Hitler. She was unable to think that Penny might not have sat with Duane for a reason like, say, she disliked Duane personally; for Aggie, rejection of Duane immediately leaps to racist.
Has Aggie grown up since then? Most definitely. But her core trait, her idealism, still rules her. As Penny and Aggie's relationship is not what Aggie wants or believes she wants- and for what it's worth, I cannot help but suspect that she is right in what she wants, that her need for an oasis is genuine- she is unable to think of the third path, that of "work on our problems, together." To her, it is either get rid of Penny or stay in a relationship you don't want. Those are her choices, because that's who Aggie is. Idealism has always held hands with extremism; they go together. It is from fire that light comes.
Penny, however, is more grounded. Penny is almost ruthlessly practical; she's good-hearted, but in a much less broad and much more defined sense than Aggie. She'd throw someone off a metaphorical cliff to save a friend, and doesn't mind giving aid when it won't hurt her to give it (as seen with the makeover attempt), but she's not going to go toss money at the poor simply because "somebody should". In a fantasy novel, she's the mercenary character, probably a straight-up warrior with dirty tricks up their sleeve; in a space setting, she's Spock. As an example, see her treatment of Sara after her coming-out. Penny saw no problem with lying about it, with passing as straight. It would be practical. She also was worried when Sara did her makeover, as she was afraid it would attract problematic attention. Her focus was never on that Sara was gay, or being true to herself- idealistic concerns- but with what other people would do if they found out and what Sara could do to hide from such trouble- practical concerns.
Has Penny grown up since then? Most definitely. But she is still practical at heart. Her concern for colleges is that it's coming up, and they have to deal with it. Aggie hadn't even considered it yet. Penny considers the options that Aggie won't, the range of grey, but that also hurts her. As she is now, Penny will never accept the value of Aggie's ideals, because Penny is too practical; she sees grey as best and sees no reason to consider white or black, either one.
To round out the metaphors, Aggie is a firestorm and Penny is a serpent. Aggie has periods of great and furious activity, but she also needs rest, and calm, and her own actions burn her sometimes, leave her with ashes. Penny, meanwhile, simply lives, experiencing life as it comes and trying to organize an optimal solution for herself and her loved ones, caring much less about anyone else and carrying plenty of venom if she has to bite. Aggie is a fundamental force of nature who cannot conceive of being anything else and Penny wants to see if she can tip the scales in her favor too much to wonder about ideals.
For what it's worth, I hope the coda isn't the 2020 Penny and Aggie. I saw that future. I would rather see another one. Personally, hoping for Penny/Sara. Penny and Sara would make a wonderful, tight-knit couple with much more in common than Penny and Aggie share. Aggie I am less certain about. Of all people there, the closest in terms of idealism is Charlotte of all people on the green earth, so that's clearly a no. Of pillars, the most stable member is taken by her boyfriend, and most other choices make no sense storyline-wise, except Lisa, who I'm waffling on. She might be what Aggie wants, but I feel she lacks a true enough empathy to be Aggie's rock.
In some ways, I suspect these last few strips are T's way of showing us that, despite our fanly years of fantasizing about it, our two favorite characters in the strip are not good romance options for each other.
STEP BACK! I TAKE LARGE STEPS!- Miles Gloriosus