The body abhors a vacuum, as they say in medicine.
The wound will heal, scar tissue will form and close the gap up over time.
They might be left with horrific long term problems like recurrent UTIs, kidney infections, adhesions, and obviously they are infertile, inability to orgasm, and so much more.
As far as I know post op TW have to dilate for the rest of their lives.
It really isn't a vagina despite what they are wish to believe and are led to believe by their surgeons. They often become patients for life due to complications from the surgery.
And yet there are many who continue to extol the virtues of these surgeries to young confused people through channels like YouTube (eg Jammie Dodger, a FTM trans person who almost died and has had horrific complications yet continues to promote this surgery to their young followers).
The surgeries are sold as making them complete but there are MANY who regret them.