Allowing those who launched the takeover of City in November 1970 (when we were holders of 2 major trophies) - Joe Smith, Ian Niven, Simon Cussins, Michael Horwich and Chris Muir - to talk with existing board and progress their takeover of the club.
That led to most of that group joining the board; they split the Mercer-Allison highly successful partnership; brought Peter Swales in; they supported Swales for most of his City career; turned City into a club that was losing money year on year rather than one that was making a profitable; led to City being a failed club and fallen giant; and also some of those same people talked Peter Swales into bringing back Malcolm Allison.
Without them who knows what would have happened, but it couldn't have been much worse.
As for Thaksin - I strongly disagree that he was one of the biggest mistakes. City could have gone under without his arrival; he took the club into clear ownership (rather than thousands of shareholders - that may annoy me personally but it actually ensured the club could be sold to the Sheikh); he brought in Sven instead of Hughes (who the existing MD claimed he had wanted); he brought in Garry Cook who fought a few very important internal battles and - this I have on tape direct from the Sheikh's immediate advisors - Cook was the most impressive thing about MCFC when they started their negotiations.
Thaksin made money out of City and did make odd demands at the time of the sale (but I know he's not the only man to have made money out of City and made odd demands when selling!) but his period was vital in the transition between the failing club we had and the one we have today.
EDIT: I'd also like to add that Thaksin did have a longer term plan for City. Not 20 years maybe, but at least 5. Anyone who sat in meetings during that time (which I did) will have heard him talk of his ideas to turn City into a global club (some ideas were very crazy, but others could have worked).
It was only when his assets were frozen that he struggled financially and was unable to do what he had planned to do.