Do the majority of people watching these actually pay close attention to the greater story arc of the infinity stones and Thanos? I mean, sure, they're aware of it but that whole thing has been shown thus far just through small cameo scenes and some short dialogue here and there. Thanos has had like what, 5 minutes of total screen time in the entire MCU thus far?
I doubt the larger audience is going to be all, "oh no, that's it with Thanos and the Infinity Stones, I guess there's no point to these movies anymore!" They're just there to see good action flicks with memorable characters, and if Marvel can keep up with that aspect, they'll be fine.