No, the notion that Trump is struggling the way Biden did is not pure fantasy. Yes, there was the debate in 2024 but there was also the debate in 2020, where many/most feel Biden won the debates. The evidence would seem to be that Biden was sharp in 2020 and he slowly declined over his four years as President, and by 2024 was reportedly showing signs of "Sundowner's Syndrome," that he was sharp during the day but had issues in the evening and night hours.
The facts are the Trump is showing many of the same symptoms Biden showed during his presidency -- and Trump being sharp in 2024 doesn't preclude the fact, much like Biden, that Trump is in decline. The other point, though, is that Trump keeps bragging about how well he does on dementia tests -- though he tries to claim they are intelligence tests and that he does better than any president before him would be able to do. Despite Trump's claims, it is a test designed to show if a person is suffering from dementia. Assuming Trump is being honest, then it would appear he does not have dementia at the moment. But the real point is this, they keep giving him the test. These dementia tests are not something given to just any senior citizen; instead, they are given to people who have shown some signs of dementia. If the person passes, they don't give them again unless the person shows more signs of dementia.
I'll agree Trump isn't terminal but, beyond that, I don't think we can say anything with certainty. We know his health reports are edited for public consumption. One example I saw was how he had high cholesterol a year ago, and so they put him on rosuvastatin and ezetimibe. This year, while it was reported that Trump's cholesterol has improved, it didn't list the rosuvastatin and the ezetimibe that he is undoubtedly still taking. It also listed his "regular physical activity," which seemed almost like a joke. Trump has stated that he does not believe in exercise -- and his exercise is largely "playing golf;" though for him that is being driven to the ball, hitting it, getting back in the cart and being driven to where the ball was hit to, hitting it again, etc.
We don't know what Trump's actual health really is, other than he is in good condition for his age and he seems to be showing signs that could be early signs of dementia.