They all get bored when the shareholders want a return. I give Mercedes another two years before they hand the team back to Brawn to run indenpendtly unless they start getting some success.
You're probably right - corporate objectives can be revised on the fly when profit margins are hit hard enough - but I remember that Mercedes-Benz returned as a constructor in order to get on terms with Ferrari in the high-end luxury car market. That's not a short-term goal and they can't advertise their way to the top: it depends on the cache that winning in F1 brings.
Of course, pouring money into a bottomless pit can't be a long-term commitment either so, sooner or later, something has to give. Five years doesn't sound all that long to me to commit to something as serious as stealing Ferrari's prestige but then the world is full of five-year plans.

