When the penny dropped that although Allardyce had done wonderfully well with his innovative idea of rescuing old quality players whose careers where on the slide, for one last hurrah in the Premier League, that the plan of paying massive wages to players at the end of their careers (and thus with no resale value) was completely financially unsustainable and would require a massive (and costly) rebuild of the squad.
Sam was smart enough to get out at the top but whoever had to follow him - Lee/Megson/Coyle - had to rebuild at short notice - and for players in their prime (in order to step into first team places) - which are expensive in terms of both transfer fees and/or wages paid!
I guess in retrospect the blame should be laid at Phil Gartside's door, as there doesn't seem to have been any planning ahead for what to do when the likes of Okocha, Djorkaeff, Hierro, Stelios, Campo, etc, had to hang up their boots.
It might be a harsh assessment from me but clearly the wheels came off not because Allardyce left but because we could no longer sustain the financial requirement to replace the quality of players that left for free around that time or thereabouts and also by that time we were starting to run up pretty big debts.
We lived the dream under Sam, I guess - and been paying for it ever since!