I've seen a lot of people say that the Fantastic Beasts series should have been just about Newt and his creatures instead of that and the Grindelwald plotline. I strongly disagree; I like that they added that plotline. You can't make five movies off of creatures; that would get redundant and boring. It's good that they added this plotline in.
For one thing, not everyone is an animal lover, so having an entire series just on animals would have only reached a niche audience. Adding the Grindelwald plotline helped widen the targeted audience, add more to the story, and bring more awesome characters into the story.
It also helped with Newt's character development. His goals changed every movie, which is how it works in real life. We don't have just one goal we aim for our entire lives. At some point, the goal gets reached, and we find new goals to work towards. And Leta's murder is what prompted him to join the fight. Grindelwald made it personal, so this became Newt's personal goal.
And while Newt had his own goals, he also lived in a society. When you live in a society, you keep other people in mind, too. Being a main character doesn't mean only doing what you want. You have your own things you want, but you help other people, too. He didn't want to pick a side at first, but at the end of COG, he did. That set up his goal for the next movie. And added to his character development. And the effect of the goal wasn't only for society's benefit; it was for his own, too. Him being a part of society meant he would have been affected alongside the others if the goal wasn't reached.
And his creatures still played important roles in the whole series. So his own interests weren't tossed out.
So yeah, I think they did fine with adding the Grindelwald plotline. There were other issues with the series, but this was not one of them.