Not sure about "kicking himself" or whatever, it really depends on the circumstances and exactly what was said at the time. It's pretty unfair to make assumptions or speculate without knowing all the facts. With hindsight, of course it looks like a *very* bad decision but that doesn't mean that the real facts were actually known at the time that decision was made.
A few weeks prior to any large festival is clearly not the time to start making major changes and cancelling existing contracts that have been in place for absolutely ages. As I said somewhere on here at the time, I wouldn't expect GFL to back out of contracts just because something better or preferable had turned up no matter who they were or how much more I would have preferred them to what was already announced. Much as I would have been delighted (understatement of the month) for Gilmour & Co to have been there, I do honestly think the "not now but definitely another time" was the 'right' decision providing that the real reason for Gilmour approaching them in the first place wasn't made absolutely clear. I somehow doubt that all the facts were known at the time because I'm not sure that Wright (and/or his family) would have wanted too much said to too many people apart from anything else because it would no doubt only have been a matter of time before it was all over 'the front pages' and so on. I would also suggest that the lack of any real information about his illness even now sort of justifies that thinking as well.
I'm fairly certain in my mind that had the full story been known at the time then something could (and would) have been done to make it happen without needing to cancel contracts etc. For instance, clipping only 5 mins off each artiste and saving a little time in changeovers would easily have found enough time to sneak in a very reasonable and completely acceptable extra and unprogrammed performance on the Pyramid stage. It would perhaps even have been possible to get an urgent licence amendment to use the Pyramid on the Wednesday or Thursday evening for just the one special performance given the circumstances. So with that in mind, why didn't it happen because it would have clearly been in the best interests of ALL parties concerned to do something like that ? I can only suggest that either all the facts weren't known at the time or money, ego and contractual issues got in the way of 'doing the right thing'. It's sad. Very sad indeed given what happened so soon after that time but you can't turn the clock back now and I very much doubt that the real story about what exactly went on will ever be known ... just speculation various and no doubt a fair old amount of bad feeling all round.