Thus far, the Netflix superheroes have lived pretty seperately from the other heroes in Marvel's universe. They will frequently reference events in The Avengers, but otherwise they are pretty distanced, especially by tone.
It seems we shouldn't expect The Defenders, or specifically Luke Cage to turn up in a movie any time soon. That is according to Mike Colter who plays Luke Cage. Calling into Breakfast Club Power 105.1 FM (via ScreenRant), the actor explained that besides the different tones, the schduling would probably be the hardest part. He said:
What we do is really unique…
We're adult oriented. We're not PG-13. We're not really for the mass audiences, crowd pleasing, family oriented. We have sex scenes. We have adult situations, and while I think it'd be nice to be in the films, I don't know if we want to dilute what we're doing that makes us very unique.
That being said, you know Marvel, if they can make it work I'm sure it'd be nice, but it's really scheduling. They schedule movies years in advance, we're shooting TV series year round. It's very tough to just kind of make things just work the way you want to because it's just so much going on.
I don't think it is the end of the world if the Netflix characters don't come into the cinematic fold, but I also think it would be cool. Those characters can go out and do things that are PG-13 and it not dilute their brand, but Colter is right. There are a lot of moving parts.
Still, it would be nice to have even a passing reference to The Defenders in Infinity War. Or better yet, one of the bigger movie stars turn up in one of the shows.