If it really is a Ponzi scheme with unsupportable excess of fees it should be ripe for disruption. There are legions of well educated, talented and experienced lawyers out of work or under employed. It is relatively easy to set up alternative provider groups of one, a few, or even several dozen attorneys without the fixed overhead and bureaucracy of the big firms that could provide services that meet then needs of the lower ends of the market at lower costs, and change the existing paradigm. Classic Clay Christensen disruptive innovation should be a natural.
But it hasn't happened to a great degree. There have been failures of big firms, but not wholesale destruction of the big law paradigm.
Maybe, just maybe, there are legitimate needs by clients, primarily large corporations, that require the resources of large full service law firms and that can not be easily met by other provider structures. While competition and pressure on costs and fees continue to drive out excess, the basic structure of big law remains because there appears to be a need they fulfill better than anyone else.