Dark Phoenix was trashed by critics, and so was The Secret Life of Pets 2, and those movies were the biggest competition KOTM had heading into the weekend. This might give the film legs, and help it make more this weekend than it would have (since it is clear RT scores matter to the general audience).
This film made less than any other Monsterverse movie so far because WB decided to release it in a very packed time (I have no idea why, it was stupid of them). This movie had major competition from movies like Aladdin, Rocket Man, Ma, and John Wick 3 in it's opening. And this weekend it is being put against Dark Phoenix and The Secret Life Of Pets 2. I believe if this film was put up against smaller films, or released earier in the year, it would have done a significant number higher at the Box Office than it did.
I read an article that stated Legendary and WB were not sweating after the low-ish opening of KOTM, because the international numbers are what's really important, (especially China, which is started to have a larger market for movies than the U.S.) and it has still not even been a week yet the film has already made its budget back.
Godzilla vs Kong is sure to bring many newcomers in, and its release around no other big movies yet, and the name GvK, is all setting it up to be the biggest film in the Monsterverse.
I believe it is still surely possible for there to be more movies after GvK in the Monsterverse.
and, like G. H. Gman said, TOHO just invested in an L.A. studio, so they are either going to be co-producing with Legendary, or they are trying to get into more markets to show there films to a larger audience.