Our success/defined end/aim is and has been for some time Premier League survival. The high end finishes have gilded it a bit, but the club is not equipped for anything grander than that - for reasons that are obvious and financial (look at the year we qualified for the CL: there was no reasonable effort to buy in the type of talent that could have took advantage of that opportunity; and look at the way in 2009 we'd achieved a very good PL position plus a cup final appearance - it was met not with more expectancy of taking the next step because that would have cost money, rather it was met with cuts).
In that context the plan on Moyes' departure was to retain PL stability plus give a new twist of a change in the style of play that would not only achieve stability but give the organisation a new identity. Martinez's appointment has to be seen in that light, and, similarly (as Steve says above) any possible threat to his status as manager has to be seen through that prism too. In short: getting shut of Martinez would not only entail a loss of finances in terms of compo, it'd mean an organisation having to go through the Herculean task of creating a new vision of its future identity again. I dont think there's remotely the will to do that unless it is a completely dire situation in the coming couple of months.