All I know is that the guy popping off some answer to a question and then going off on a bunch of tangents (at best!) is not the same guy who back in the summer of 2015 was still giving succinct and on-point although self-serving answers to press questions during casual encounters.
At that time he was proving entirely capable of just moving on from the encounter. By that I mean he wasn't popping up later with a tweeted tirade based on hours of subsequent internal obsession about having been asked some question or having his response questioned on a follow-up.
Sure, that reads more like narcissism taking him south than dementia, although sometimes rage is part of the picture when someone's aware of losing cognition, but whatever it is, it has got worse. So we can stick a label of some sort on that or not --and I'm not a physician-- but it doesn't really matter to most of us out here because
1) whatever Trump's problems are have been progressing since 2015, and
2) it's not reassuring to have a US President consistently demonstrate inability to stay on even his own political message. All those "theme weeks" the WH staff prepared for him to roll out? He was advised not to take questions but when he did, he invariably ended up stirring some pot with a reporter that stepped on the theme by time the scheduled presentation was over.
How's he going to do on the campaign trail? He's more unfocused now than he was during his highly inappropriate departures from prepared remarks at that Boy Scout Jamboree a few years ago. Imagine how GOP state chairs feel about this guy coming to their state on the campaign trail this fall, virtually or in person. He's a dice roll for them now every time out the box.
In theory this should please me, since I lean left. It doesn't. He's the US President and might even get re-elected. Yet he's a guy whose performance would not be tolerated if he were an officer of any publicly traded corporation. The fate of the nation rests in his hands today, and if he's not actually at the helm then we're not clued in on who's really minding our store.
At the moment Trump is grossly underperforming because the violent incidents in Kenosha and Portland are not being addressed appropriately by the White House. He's trying to make a partisan campaign issue out of isolated breakdowns in an otherwise mostly peaceful nationwide expression of weariness with systemic racism in police behavior. Past that he's trying to grandstand some EO-supplied fixes to covid-19 economic fallout, instead of managing to get Democrats and Republicans to pass legislation for an adequate followup stimulus program.
But mostly Trump's proving unwilling to focus on healing rather than dividing us over the fact that all of our lives matter, no matter if we personally camp out these days with "black lives matter" or "blue lives matter" or just wonder how we got to a place where a US President can turn up so useless during times of crisis. Our bottom line problem with Trump is that none of our lives really matters to him.
Sad to say that, and part of his inability to respond appropriately to racial injustice, covid-19, economic uncertainty, etc., is probably due to his illness. That's sad but certainly not a reason for him to be returned to office for another four years. I don't think Kenosha or Portland or even the tiny town of Bainbridge not too far from me (where a peaceful demo against racial injustice raised some nasty divisions later on ) can take the lack of national leadership on that issue that Trump has shown us.