Having gone and looked in detail at the speech in question, for instance here:
www.townandcountrymag.com/society/tradition/a26708102/prince-harry-we-day-speech-transcript-2019/
I think pulling out a sentence from a MOTIVATIONAL speech and using that as a straw man to get excited about and refute is rather missing the point.
Yes, there have been campaigns throughout the later 20th century, I assume by this is meant campaigns to make the world a safer, healthier, more peaceful place but hello, are you listening,??? because the world is in a worse place than would have been imaginable 30 years ago. Action is needed now and young people need support and encouragement. As Harry says climate change is the biggest challenge of all, never mind inequality, poverty, rampant corporate behaviour, tax evasion, political corruption, wars, fake news, cyber crime, the resurgence of Russia, political tensions in the Far East between China and Japan, anti nuclear treaties are being torn up and limited nuclear war is being imagined, the scramble for territory and influence as the ice melts at the North Pole and access to oil and free passage becomes critical. Massive changes are happening now, stop looking backwards, the world is in a mess. Whether or not you think you or the UK are in any way responsible is actually irrelevant to the scale of the crisis facing us. If I were young I would be very depressed as studies show the young are and that is what the speech in question was attempting to address in a highly relevant situation.