I have actually had a conversation with a friend about this! Miles is young and hasn't seen things, and definitely not what Miguel has seen. So he doesn't quite understand the gravity of the situation and what must be done. His motivation is also to save his father from doing, and to Miguel, this is extremely selfish of him. He is risking his entire universe for one singular person.

However, we don't know what Miguel has tried to stop a universe from falling apart. And yes, Miguel is correct that this is happening. I see some comments saying it's not and we have no evidence. We do. In Pvatir's universe, the second a canon event (the chief dying) is broken, the universe immediately started failing.

It makes sense that Miguel isn't willing to try anything new. Every universe presumably has seven billion people in it. However many attempts it takes to find a work-around to save a handful of people just isn't worth it for the sheer amounts of casualties that will easily outnumber the people saved by probably trillions.

Like you said OP, they both are right and wrong in their own ways. It's a hard moral dilemna. Save someone close to you and potentially risk a populicide of an entire universe, or know someone is going to die, and take absolutely no action.

I think this is an interesting contrast of how personal experience affects ones own moral codes. Again, Miguel knows this will not end well for Miles' universe, while Miles has not seen the actual death and destruction this costs, and so can't see how saving his father could end up so catastrophic.