I don't necessarily agree on the simple fact that KoTM only really didn't make money because some idiot decided to release it on the same weekend as Aladdin which Disney went ham-fist on advertising for (way more than they did for The Lion King which was way better IMO). KoTM had noticeable flaws but it was hardly a mess of a film.
KoTM was a Toho movie with an American paint job thrown over it so it's going to have the silliness and ridiculousness of what Toho films offer but Americanized. The modern casual audience isn't going to realize that and the only Godzilla films they've most likely seen are the shitty '98 movie and Godzilla '14 so they expect this movie to be on par with modern classics.
The plot was one thing people complained about but in a franchise that has 30 films and has been around for half a century, it's difficult to come up with something new and original. The plot did not bother me as there have been more hilarious plotlines in Godzilla films before that make no sense like Godzilla vs Megalon, Godzilla vs Gigan, and the first Godzilla vs Mechagodzilla. And hell, the original movie's concept has been remade like 4 times by now.
The human characters are also another complaint with KoTM, but come on. Name me ONE human character from any
Toho Godzilla movie people gave a shit about and actually remember the name other than Miki Saegusa from the Heisei Era.
People want better humans but if we get that, they'll complain there wasn't enough monsters. Godzilla vs Kong can be 2 hours of just both of them fighting with 0 humans at all and people will still complain that "there wasn't any humans for me to care about!" even they got what they asked for in more monster action.
KoTM didn't perform as well as we thought. The movie was going to release in early 2019, and that would have been fine and probably for the better, but they ended up pushing the movie back to attempt to compete with the juggernaut that is Disney and that backfired. People were fatigued from that movie season we had from April-July and that impacted KoTM.
Also something that I see NOBODY talk about when it comes to modern Godzilla that is a problem is the generational gap. Me, you, and other kaiju fans grew up around watching Godzilla movies on TV or renting them from Blockbuster and we had the PS2 and Xbox games to play when no other Godzilla media was available. Access to Godzilla was easy and everyone in our age gap enjoyed it.
Kids nowadays have no Godzilla games, no stores to rent Godzilla movies, and are growing up watching The Avengers and other Marvel/DC movies rather than watching Godzilla like we did.