Petrucci and LaBrie are what makes DT, well, DT - Petrucci with his songwriting, his style, his attitude towards the band and the fact that he's always been there since day one. There are out there guitarists who could technically replace him, but only JP can write songs that make DT the band it is.
LaBrie is the voice of the band. Freddie Mercury reincarnated wouldn't feel "right" with DT, he's been there an entire carrer minus the debut album and no matter who you get, it wouldn't feel right. You could find a singer who could nail Learning to Live night after night, but a singer that would feel like he's born to sing DT songs? hell no.
Jordan's leaving would be a huge blow, and would change very drastically the dynamics of the band, but as long as Petrucci is still there, it would still sound like DT. After all Jordan wasn't on Images and Words and Awake.
Myung... the man is so shrouded in mystery, and I'm not a musician or bass player anyway, so I can't have a proper opinion on how much his presence is vital to the core of DT.
Mangini, sorry, he's the replaceable one. No disrespect to the guy, I like him and I hope he stays until the end of the band, but he's with the band since only 2011 anyway. DT went on without Portnoy, they could as well go on without Mangini.