let's not forget how the seasons post-book often brought stuff from the book with no explanation to the audience that hadn't read it, but then the stuff from the book that seemed like it would be a bigger deal (like seth) was never brought up in lieu of a conclusion that takes the absurd conclusion of "magic = bad!" and runs with it to defeat the main threat and many of star's own allies in the process
if there was some grander statement like with the animorphs ending which also ends with a mostly tragic ending for all of the characters, i could maybe see their point but its attempting to be framed more unambiguously happy. characters that would die as a result of this say they're fine with it even though it doesn't really make sense. the cost of war is never reflected on, as ponyhead fills us in on characters living happy lives as the result of this and the bad ones that died are shown to be in a world that doesn't give a shit about their lack of existence in the story anymore.
all of this takes backseat to a romantic relationship that never worked to begin with. you really think star and marco are supposed to be a happy couple together when all of their past relationships died thanks to a lack of communication of what they wanted? they're framed like soul mates but much of that context is from the blood moon plot point that according to this show, is false and doesn't matter.
star killed hundreds if not thousands of innocent characters that relied on magic to survive. the message is not that star is an awful person for doing this and should have thought this out more, it's that so she can get together with her shitty awful boyfriend who can't communicate and elevated pretty much every woman in his life on a pedestal with one exception, who he treated the absolute shittiest. he didn't learn and won't have the time to. good thing things went star's way, it sure didn't have bad repercussions for everyone else (except it did, we literally see monsters attacking innocent people in the merged earth and mewni world and a lack of magic killed all those characters).