I'm personally not a fan of the Mistoria pairing. I'm not here to do shipping bullshit. Cats aren't naturally monogamous. All the ships can be canon at once.
I want to go into why Mistoria is a thing in 2019. It wasn't because they were trying to make Mistoffelees more straight. I also don't think it was because they were familiar with a few productions in the early 90s that did it and decided to copy them, because the Mistoria in Paris and Zurich isn't really the same thing we're getting in 2019.
Victoria's pas de deux partners have varied between productions because the pas de deux is about Victoria. The tom is basically a prop for her development in this scene. It literally does not matter who it is. It doesn't even mean you need to see Victoria/Whoever as a couple. Once again, no monogamy here.
Victoria has, in different productions, been paired with Admetus/Plato, Tumblebrutus, Carbucketty/Pouncival, Alonzo, Coricopat, and Mistoffelees. Now, what makes Mistoffelees different from everyone else on this list. No, it's not queer-coding. It's that he's a main character.
The movie was clearly trying to mainstream the show, make it make more sense. It didn't work, but that's what they were doing. Cats has a lot of characters who are around the whole time and technically do things, but aren't major characters. Nearly every single tom listed above has been cut from one production or another. A good amount of what they do can be given to another character. All those toms appear in Cats 2019 only as background characters and it was assumed that people wouldn't want to see Victoria dance with some rando from out of the background. So they picked the one tom she'd been paired with before who was focused on at any other point.
Mistoffelees is a prominent character who interacts with Victoria. That's literally it. That's why they went with him. It had nothing to do with shipping or heteronormativity. It was a practical matter of keeping the focus on the characters the audience recognizes. That's it.
As for making a whole romance arc outside of the pas de deux, I don't like that, but I wouldn't like it any more if it was with anyone. Once again, no monogamy here. Sexual tension, hook-ups, but not this. It isn't that it's Mistoria, it's that they made monogamous romance prominent at all. Even then, it's not 100% clear that it's monogamous though, so that might not be too much of a problem. They put a romance front and center because it was most likely assumed that a mainstream audience would want one. A lot of movies have romance arcs that they don't really need.
The actual problem I have with Mistoffelees in 2019 isn't that they "made him straight", but that they took a prominent dance role and removed most of the dance from it. Victoria is still as much of a dancer as she is in stage productions, but she's paired with someone who can't really keep up with her.
In Paris and Zurich, Mistoffelees is still a major dance role, just like in every other production. It just happens that one of the dances he does is the pas de deux. In this productions, both Victoria and Mistoffelees are silent characters who mainly communicate through dance and gesture, and they get a big dance together. Those shows most likely picked the pairing for the aesthetic, putting the two best dancers together, along with a visual contrast of black cat/white cat.
Cats 2019 took a dance role, removed the dance, and didn't really replace it with anything, making the character not work in general, while also expanding his role. There's also the presence of Munkustrap, played by a dancer, so he compliments Victoria far better than Mistoffelees does.
Adding the fact that Munkustrap takes some of Tugger's usual role in Misto's number: Munkustrap/Mistoffees/Victoria 2019 OT3.
