Then proceeded to win the election. Did you forget about that part?
He did, but that doesn't necessarily prove the report wrong. Let's consider this for a second with another example. Someone fills up his car, drives non-stop all day, and is told he is riding on empty. He says fake news, the gauge is wrong, he has much more gas than people realize. He makes it to a hotel, sleeps, wakes up the next day, gets in the car, and it won't start because it's empty.
Sure, he made it to where he was going, but it didn't change the fact that the car still ended up running out of gas. Likewise, Trump winning the election doesn't prove much. I suppose if Republicans run the same kind of campaign on the same issues and win handily in 2032, you might have a point.
Anyway it's pretty much a fact at this point that Democratic Hacks have the memory of a goldfish, so we shouldn't be surprise that "Demographics are destiny" nonsense is back, after it had been complelty discredited only just over a year ago.
Was it? The only thing I think was discredited was the opinion of when exactly the demographics=destiny prediction would materialize. The fundamental idea is still the same - that Republicans keep digging in with the same shrinking group of voters while not only failing to make inroads with the growing ones, but alienating the ones that they already had. Nothing really has changed. They are still losing out with voters they need to be winning to secure an electoral future, choosing instead to just hope things magically work out on their own.
Personally, I don't think the GOP is going to die out - that seems silly to me given how long both parties have lasted, but it seems to me that a party that is consistently alienating most/all the major growing demographics and doing nothing to change is probably going to have a tough time in the future, when much of their aging base has aged out of the electorate.