Patted a SR-71 on the nose at Castle Air Museum and was amazed how small it seemed.
Answer to why it was retired is straight-forward, cost. The fuel alone was ridiculously expensive; a quote that sums it up- "It would have been cheaper to use single malt scotch". Assuming that any country that could shoot down a U2 could also shoot down a SR-71, having them do 2 000 mph is not needed. Also, satellites are a little better these days https://vimeo.com/130889259
I realise that this video is far, far below the spatial resolution of even a 50 year old spy satellite, it's still indicative of new directions that are now possible.