Black Widow serves as a prequel to Avengers: Infinity War and a sequel to Captain America: Civil War that has Natasha Romanoff on the run and confronting her past. As she tries to stop the Red Room once and for all, she also encounters her surrogate family, including her "father," the brash but ultimately warmhearted Alexei Shostakov, the former Russian superhero known as the Red Guardian. Although he is still the super-soldier Natasha knew from childhood, one theory proposes a crushing reason why the fading symbol of Russia is no longer as powerful as he once was.
When Natasha and her adoptive sister, Yelena Belova, reunite with Alexei, he is being held in a Siberian prison that looks barely capable of containing a super-soldier. However, Reddit theorist grambocrackah suggests Alexei is slowly being killed by the very Super-Soldier Serum that transformed him into Red Guardian, and only the prison's physicians were keeping him alive.
The Redditor points to an apparent plot hole in Black Widow created by the apparent proximity of the prison to St. Petersburg, suggesting Alexei otherwise should have been able to escape and hide out. Yet doing so would have meant ending medical treatment, leaving him weakened and on the verge of death.
That could also explain why he knew how to find Melina Vostokoff so quickly. She was likely the one who treated him while they were undercover in Ohio. Knowing that his own death is imminent, no matter how hard he tries, it makes sense that Alexi would act as selfish as he does after reuniting with Natasha and Yelena. Ultimately he knows that breaking out and dying a hero is a more noble fate than spending the rest of his life in prison. But in knowing he will die, thinking of himself comes more naturally, causing him to forget about the feelings of his "daughters."
The theorist then delves into the most obvious clue to his diminishing health: his appearance. Unlike Bucky Barnes or Steve Rogers, Alexei's body is in terrible shape. Not only has he put on more weight, but his dental hygiene isn't great, and his overall complexion isn't great either. Having a Super-Soldier Serum makes it common for these bodily functions to never waver, and keep the user at their best self for years. Even an older Steve Rogers looked younger than he actually is. So, logically, it makes sense that the series designed to make him his best self was also losing its potency.
Although a more arguable point, there's also the topic of Alexei's mental state. While in prison, he finds himself focusing more on the past, and following his breakout, he can't seem to stay in the moment -- to the point that he doesn't even receive an earpiece during the infiltration of the Red Room. Although this could be explained away that he was always like this, and he has never been the most reliable with technology, it makes more sense with the theory when seeing how efficient he was in the 1990s while in Ohio. Ultimately, it seems more likely that his obsession with being a hero, combined with a deteriorating mental state, created a man who is in the present but still living in the past.
By the end of Black Widow, Alexei is the hero that he wants to be and someone worthy of being a super-soldier. However, even with no confirmation on the validity of the theory, it nevertheless raises the possibility that his time is limited. Should he appear again in the Marvel Cinematic Universe, it will be interesting to see how he adapts and grows as a hero and if it will further add to the idea that he's in a race against time.