The metacrisis isn't an illness, it's is made-up space magic. RTD can put whatever rules he wants onto it. It makes a sort of sense (in a wibbly-wobbly sort of way) that a child would act as a sort of pressure release.

Them "letting it go" at the end was a bit of a silly copout, but I chalk it up to the DoctorDonna (and DoctorRose?) having capabilities the Doctor couldn't fathom alone. She/they just needed fifteen years or so to ruminate on it, like a complex calculation that takes years to compute.