Keith leaving had nothing to do with the poxy 3 man rule, as remember Uefa's regulations decreed that clubs could field only three foreign players plus two "assimilated" players who had come through their youth set-up which Keith did, and as for him "having" a better career under Fergie, you have no way of proving that as, hmm Lee Sharpe, Fergie couldn't control his party lifestyle, just like he couldn't with Yorkie either hence why both were shown the door, so to say he would have thrived under Fergie is madness gone mad.
He was let leave due to Beckham's rising in Fergie's eyes, and don't forget just 10 months later a ruling came in by the EU not little UEFA in terms of those two, in stopping that 3 player ruling, which was deemed a breach of the 1992 Maastricht treaty and saw the end of it with the Bosman ruling Dec 95.