I thought it was great, right before the last few minutes when, as others have pointed out, unnecessary preaching was added. The big issue was, it was claimed to be "One possible future", but then The Doctor goes on how about how Humanity needs to face facts and change (but doesn't say what, because it doesn't really know itself, so its just leaving it vague), putting the blame squarely on what people know when they watch the series, and that for us would be Global Warming stuff and what not now (as its current affairs). Much like how Princess Diana means squat to most people now. It's just a different thing.
It would have been so much better if The Doctor actually acknowledged HOW and WHAT happened to that timelines Earth. And then solemnly and sadly lament at how silly and stupid Humanity can be sometimes at not changing to stop it. And if need be, she's seen other Earths where similar things happened. This makes it clear that Humanity is flawed, and that it has a tendency to screw things over for itself, without making the audience fill in the blanks themselves. This gives the same message that people need to take more heed of the world around them, without needing to fill in the blanks themselves.
This is also why certain films and shows don't age well. Because instead of relying on the story itself, it's relying on actual current affairs to make it work. Would Schindlers List really be that meaningful and powerful if there was no World War II and the attrocities that occured?
A shame, the majority of the episode was decent in my opinion, wasn't like last Season at all. Then the last part happened and it all went to last Season again.
Still needs work Season 12 I say. But again, not impressed enough to give Doctor Who the all clear, and if this continues, I doubt I will. But for now, will watch next episode to see how it goes, it's earned that so far from me.