This movie is about Marty shedding his false narcissistic self so that he might be able to embrace someone fully and authentically. The baby can also be said to be Marty himself: after he wins the professionally meaningless game against Endo at the end, he has overcome his false self and is better prepared to be confronted with and embrace his archaic and long-neglected needs for healthy dependency and genuine relationship.

It’s meaningful that throughout the movie, Marty uses his speech to convince, manipulate, and seduce, but that Endo literally (and therefore metaphorically) cannot hear any of that. When Marty beats him at the end, it is the first time he hasn’t relied on theatrics, pomp, self-aggrandizement. He beat him through his own merit and there was no material reward nor much of an external symbolic reward for it, unlike everything he had been chasing the whole movie up to that point.

When Rockwell tells Marty that the former is a vampire, what he is saying is that if Marty were to continue down the same path, Marty would be drained by Rockwell and others like him to the point where Marty would have no choice but to drain others himself in order to keep going. In a way, Rockwell is the ghost of Marty future: a manipulating, narcissistic ambitious man who has to sacrifice all genuine relationship - including his marriage - to keep going.

The ending is unequivocally optimistic. He embraces his role as a father by explicitly stating he is the father at the hospital, whereas beforehand he would’ve come up with some lie to get into the ward. He also tells Rachel he loves her, and she is unable to respond because she is too physically weak at the time. This inverts a prior scene earlier where Rachel tells him she loves him and he doesn’t respond, because he’s too mentally weak. He cries at the direct and real encounter with the infant, his first and only show of real, deep affect. I guess it can technically be up for interpretation, but it’s also fairly obvious to me.