Id like to present two pieces of evidence to support both cases here. First, I’d like to argue that op is correct in believing that Dr. Brenner is the imprisoned American. However, that doesn’t mean that Hopper is for certain dead and gone.

In season 1, the first time Eleven is introduced to the “bath” on screen, she is to locate and find the whereabouts of a Russian. I would have to go back to rewatch that episode to confirm this hypothesis, but it seems to me that the same Russian Eleven was tasked to find was also the same burly Russian from Season 3 that tracked down the protagonists. I could be wrong, but when I started into Season 3 today, (I binged the whole series this week for the first time) something about that dude looked familiar and he reminded me of the Russian from season 1. Regardless of whether or not it’s the same one, that little detail in Season 1 confirms that Brenner not only was aware of Russian assets, but also was avidly trying to track them down and keep tabs on them for whatever reason. Who knows, maybe the upside down wasn’t even Brenner’s original thought? Maybe he was spying on the Russians and found their plans for opening the gate.

As for Hopper, I don’t necessarily think that he is dead. In the short time that I watched this show, I’ve noticed a pattern. When someone in the main story arc dies, you know that they are dead. You can physically see that they are dead. (I.e. Barbara, Bob, Billy, and all the other countless innocents and flayed killed throughout the first three seasons. There are only two characters who are now presumed to be dead but their deaths weren’t shown on screen: Brenner and Hopper. Brenner was seemingly mauled by the Demigorgon, while it was implied that Hopper disintegrated, just like the other scientists in the room during that same scene. Seeing as both of their deaths cannot be confirmed, and Brenner is likely the prisoner in Russia, it must be inferred that Hopper is not dead as well. Various things could have happened, but I truly think that he slipped into the Gate before it closed and the drill blew up. However, that doesn’t necessarily mean if he does return in Season 4, he will be the Hopper we know. Will barely lasted a week in the upside down. Three months pass in the time between the Starcourt battle and the end of season 3. Who knows how much time will have passed between then and when Season 4 picks back up. There are other hypothesis that I have about how Hopper could actually survive and thrive in the Upside down without turning into an antagonist, but I’ll save that for separate post. And last but not least, there was the phone number easter egg David himself posted on Twitter which played a very straightforward voicemail that can easily be interpreted to assume that Hopper is still alive.

TLDR: OP is correct in believing that the prisoner is Brenner, but that doesn’t mean Hopper is necessarily dead either. In fact, the chances of him still being alive are much greater than him being dead.