I can almost see this as the correct answer.
Homestuck is, IIRC, supposed to be about young teens growing to be adults. I could swear I read Hussie directly stating that somewhere.
As such, SBURB is noted in-universe to be directly involved in various ways in the process of its participants' personal growth. Presumably as they influence the creation of the next universe, a healthy, matured playerbase is literally essential to this process. It can be guessed that as part of the reproductive cycle of birth for universes, it is heavily invested in ensuring that any session is both successful, and produces viable offspring, and therefore is weighted (with all the weight paradox space temporal shenanigans can invest in inevitability) towards enforcing that personal growth in all valid sessions.
Also as such, the antagonist for a story with this theme would have to be a character who had denied his personal growth. Become incapable of it, even. A peter-pan-like figure who had never truly grown up. And thus we have Caliborn/LE, who has psychically mutilated himself to become incapable of his species' version of adulthood.
As depicted, Null and Dead sessions don't make sense in that aspect, unless they are actually special sessions geared towards different player needs. Null sessions are fated to fail without outside intervention, but presumably always receive such intervention in the alpha timeline. (And may not be intended to matter. More on this in a bit..) And Dead sessions have just been stated to be weighted in favor of a Time player, which would make sense as that is the best way to enable subsequent universes to interfere with the player. Heck, Doc Scratch even points out that LE is a known factor being accommodated for by paradox space, which suggests he's supposed to be there.
I've mentioned in a previous post that my theory was that the Troll and Kid sessions were all actually subsessions of a mega-session: that of the cherubs. That LE's session hadn't actually ended, and the real point of the story was the successful completion of his session in some manner, with the kids and trolls effectively just mechanisms for completing it in some manner. That their universes were doomed byproducts of a giant temporal reacharound intended to force the success of LE's session, and therefore possibly doomed, since the offspring of the mega-session was the real point. (Thus a null session might only really appear in such a subloop, created not to produce offspring of its own, but to facilitate the creation of a viable offspring in an ancestor session.)
But, as mentioned above, both of the cherubs killed their counterparts too early, thus creating their dead session and stunting their own growths permanently. They will never be able to successfully complete such a session... unless what you just suggested occurs!
If Calliope sprites and prototypes LE, the result will be a single entity with both their minds. In other words a normal, healthy adolescent Cherub. (Admittedly, one with multiple godlike abilities and enhancements...) They'd be able to mature the correct way, and finally complete their mega-session, bringing the whole story to a close.
So yeah, I think you've hit on something really likely here. It may not be exactly how things go down, but given that the sprite prototyping has suddenly been given more prevalence, setting such a thing up, and the nature of both cherubs' maturation and SBURB's investment in player maturation, I think it's more likely than it might seem.