When you look at the box office for Godzilla: King Of The Monsters you quickly realize it won't make as much as Godzilla 2014, whether or not it is considered a success is a matter of opinion. But when you start looking at many sequels in this genre, or even in other genres, is what could be called the sequel or sophomore jinx, that occurs.
Pacific Rim, the film that started the current kaiju trend, made $411 million, Pacific Rim Uprising approx $291 million, a significant drop, Godzilla 2014 made $529 million, Godzilla: KOTM will finish under $400 million (currently $386 million approx.), other big drop. This trend isn't just the recent films King Kong 1933 did quite well but its sequel Son Of Kong didn't do as well, the 1976 remake also did relatively well but its sequel King Kong Lives bombed, more recently Cloverfield did well but its first sequel had a drop at the box office and second sequel was shifted to Netflix. Even looking at the Jurassic Park/World franchise each film has done less than the previous film in each part of that series (Park separate from World films).
In the case of Godzilla Vs. Kong we are technically getting the third Godzilla and second Kong film together, could that combo be enough to buck the trend. While not all sequels have had the same pattern the monster or creature genre seems to have it be more the case, it is a possibility this is why we have not gotten a more definitive word on sequels to other films like Rampage, The Meg, etc. even though they were successful films.
There are many reasons why this can be happening like, the shock/awe valve and curiosity are there in the first film but isn't as much in the sequel. Also many sequels become more story focused and look at the continuing story arc going forward which may loose some, there is also issues because they aren't as interested in the human characters or character/monster development (they want monsters to be monsters for example), it's a delicate balance and it's hard to truly satisfy the audience as a whole, especially non fans of the genre.
Another more modern problem can be reviews, curiosity even with not such great or bad reviews will still allow a new film to do well but that curiosity isn't there in the same way so bad reviews weight a lot more on a sequel. There can also be a lot of dissecting of films now which with all of the special effects, story details, and such that they can can get hurt by being over analyzed.
The big questions for Monsterverse fans is how will Godzilla Vs. Kong do and will the Monsterverse continue. The first question may be helped by the nostalgia, since many fans, serious and casual, have been looking for a rematch since 1962, another thing is Kong was slightly better reviewed and received which may help. On the flip side questions of Kong's size and how he will match Godzilla may and have caused concern, yes he was growing but it has been a source of jokes since his film was released.
Warner Bros. and Legendary have done a lot to try and built to it and build the Monsterverse into something so hopefully that pays off and helps, but the poor showing of Godzilla: KOTM may have hurt mainstream interest. Toho will certainly be watching, a big box office and the Monsterverse continues, a poor showing and Toho may decide to do their own thing (which could happen anyway) and the Monsterverse may end.
We hope it does well as we look forward to where the Monsterverse will go and its continued development. In the end no matter what happens we personal are happy overall with what we got and hope fans are too.