I think it was actually probably a good power play by Fergie in the short term. There was a lot of talk that 'the old man has lost it' and that United were on the cusp of a decline at that time. I remember when we signed Rooney and Wenger described the signing as something we had to do just to keep in touch with the leading pack. A lot of fans were starting to get twitchy about where the club was heading. I think what happened with Keane ultimately reasserted Fergie's authority and ruthlessness and helped us kick on into a new era.
Many see United as this juggernaut that can financially out muscle every club. That represents a tiny part of our history. United is also the club who relied on Graham lending Sir Alex his contract so that the latter can show it to Edwards and get his payrise, its also the same club who couldn't afford to match Batistuta's salary with Fiorentina (not Real, not Juventus, Fiorentina), not to forget it is the club who forced Stam to relinquish his signing on fee else we wouldn't have been able to sign him and who nearly lost Keane on a free to Juventus cause we refused to give him the salary he wanted.
Sir Alex worked in that environment for most of his career. He was expected to punch way above United's weight. To do so, he brainwashed his player to think we're different. While Inter or Real could afford giving players better salaries, we on the other hand, were a family team. Our ethos was simple, do well, work hard, sign the damn contract even though you could earn more elsewhere and in exchange the club would always have your back. TBF he did his best to keep his end of the bargain. Sir Alex was proud that none of the hundreds of kids coming out of the academy ended up leaving us without a job. He also asked ridiculously low fees for English internationals such as Phil Nev and Nicky Butt, so that they could join decent clubs. Fletcher was handled a long term contract even though Sir Alex knew exactly what type of illness he had.
Unfortunately that deal couldn't cut it on one thing ie age. Time was something no player could beat and unless a player acknowledged that his time was up, then such 'cultural' thing would have ended up biting our arse. Keane was neither the first nor the last who overstayed. The list was endless from Robson to Keane, from Gaz right to the likes of Giggs, Rio and Carrick.
Sir Alex describe Keane as a man who refused to adapt to age. A Peter Pan wannabe who thought age couldn't catch up with him and was reluctant in adapting to it. He was also the only player at United who had the influence to challenge Sir Alex authority. Ince loved to call himself the guvnor but Keane was the only man who was able to force the dressing room to go at arms against Sir Alex's coaching decisions, even persuading Gary Neville (the closest equivalent to a Sir Alex's groupie) to lead the charge. The only way Sir Alex could wrestle his authority into the dressing room back was to first publically humiliate Keane and then kick him out. Which is exactly what he did when he showed that infamous MUTV interview to entire squad only to fire him later on. The funny/tragic thing is that Keane was so confident that he had the dressing room in his small pocket that he was the one who suggested to Sir Alex to show that darn interview in the first place and he even challenged the same players to give their opinion about it once the interview was over. Instead of having the players back, those same players ended up either walking out (Scholes, Fortune etc) or confronting him (VDS, RVN) as Keane was reduced to an old broken player thrashing insults to everybody who dared defying him. That's a humiliating experience the likes of Keane would never ever forget.
I am not suggesting that Keane doesn't have a point. Sir Alex's nepotism/favouritism + his non football related issues was quite evident and did hurt the club at times. However one can't accuse a man of using nepotism/favouritism only to then accuse him again for not using that same nepotism/favouritism to keep him at the club when it was evident that his time there was over.