See, but Cadence was/is the rightful ruler of the Crystal Kingdom. Making twilight an alicorn completely and totally destroys and semblance of alicorns being unique in any way, not when you can apparently just poof someone into one. That's the only problem I have with the occurrence.
Fair enough. But, I think the choice to allocate Cadence to the Crystal Kingdom was probably an 'after-the-fact' development. She likely wasn't born yet when Sombra took over the kingdom (considering she was still normal pony-sized when she was filly-sitting Twi), so I find it unlikely that her ruling the area was pre-planned.
Personally, since Celestia and Luna are god-monarchs, I prefer the idea of alicorns being 'born' from magic (a critical element of the Equestrian universe) rather than being conventionally and organically conceived. Meritocratic reproduction, rather than biological reproduction. (Though, now it would seem that both origins are possible.)