All other attempts (747SP, TriStar 500...) have failed dismally.
Have to take issue with you there. We had six L1011-500s, extended wing, active wing load relief, FMS. I spent three years on them and I believe they worked out well for the company. Our regular ones did not have the range for the routes that the -500 was on.
I thought that it was one of the best aircraft I flew, although I realise that that does not make them a commercial success. Never the less, I think the aircraft was a success for our company.