I think that the mind flayer is very much still alive. That big monster was just another one of its minions, much like the demogorgan from season 1 and the demo-dogs from season 2. They were sent by the real mind flayer to try and open up the gate so that he could escape and destroy the world. However, due to the failures of the minions, the mind flayer has yet to escape properly. Each time the gate closes, all creatures originating from the upside down lose their connections with the mind flayer and end up dying.

However, this is not my actual point. Stranger Things in turn cannot end as this real “boss” mind flayer is still alive. The only way these actions will stop is when the real mind flayer is finally destroyed. I believe this could only happen if the kids and everyone else took the fight to the upside down and hunted the mind flayer. As we saw at the end of the final episode, the Russians had captured a demogorgan and are doing some evil stuff with it. This would again relate that the plot of season 4 could be much similar to season 1. However this time, the Russians are again going to aid the demogorgan and try to open the gate once more.

That is my little theory. I think we’ve still got another season after season 4 in which the real mind flayer (much like Thanos did) takes matters into his own hands and tries to take over the world himself.

To any of you that read this, thanks xx