some thoughts on mettaton and spamton actually:
the idea that alphys and mettaton's scheme in undertale was that alphys would basically make mettaton's dream come true and he could become big and famous on the condition that he keep a coveted secret between the two of them that would separate him from his friends & family/old life. but he decided it was worth it, thus he became the Abandoning Abandoner... HHHMMMMM.
Of course these are different situations when you get into the finer details. Obviously, Alphys and Mettaton's scheme was earnestly mutually beneficial and derived from their strong friendship. Alphys has been the "successor" to Gaster but she is very different. I think what matters is actually that Mettaton's story is like. the 'uplifting ending' version of Spamton's.
Mettaton got everything he wanted and even a killer robot body, and for a while he made massive sacrifices and hurt his cousin, but in the (pacifist) end he gets the opportunity to make up for the ways he hurt them and hurt Alphys. He was incredibly selfish and egotistical, but he had the chance to learn his lesson and still be a TV star with his loved ones at his side. Spamton, on the other hand, got dropped by his strange someone and then it was just loss after loss. he was nothing without his beneficiary. He had burned through all his relationships without the opportunity to repair them. he was filled with bitter resentment and desperation. And what does he become obsessed with? Mettaton's NEO body in the basement.
he believes that the robot body is going to bring him salvation and liberation. but even when he gets that, it's just another ceiling.
Spamton invests in the robot body as a source of power for him to take. Mettaton NEO was "Dr. Alphys' Greatest Invention," but spamton NEO is "his own worst invention." the Mettaton NEO description emphasizes alphys' contribution towards making mettaton who he is in that moment. despite the fact that he almost immediately fails, there's something to be said for the love between friends that brought it to fruition. for Spamton, this body is the last stage in a long line of wearing other people because being someone else was the only way to get out of being him. From red suits with yellow ties to mismatched glasses and disguises, the NEO body is not a great invention, but rather a final act of self-desecration from a very lonely man.
And both Spamton and Mettaton ultimately have the same plan when all else fails: take your human/lightner SOUL and ascend.
Mettaton ultimately learns that the monsters in the underground still need him and he's okay with sticking around and letting frisk handle things. Spamton learns that he can't be anything more than a simple puppet, so he decides to give himself up and help the heroes break their own strings.
I think a lot of it is kind of glass half-full/glass half-empty mentality, where Mettaton just seems more willing to think positively about the situation than Spamton's (probably justified) doomer outlook, but also it is like. spamton being divinely and biblically screwed over by the person on the phone. spamton being desperate versus mettaton being a dreamer. mettaton isn't the platonic concept of a spam email. and in general he is a more likeable and talented person even at his worst, probably. etc etc.
I believe that it simply just makes sense narratively to think about the use of Mettaton's NEO body in Deltarune and how Mettaton specifically thematically resonates with Spamton. The characters function like two parallel lines in a way. Both characters have wanted to reach further and find themselves but have had vastly different results for the most part. I think it shows how Undertale is a story of unbridled, resilient hope, the freedom of choice, and the impact of your choices, vs. Deltarune's framing as a story with ONE ENDING and where the characters keep feeling like no matter what they do, they can't change where they're headed because of the prophecy.
