Dedicating a team to Mark Cavendish is a good plan for Astana, because every plan is good on a team that right now doesn't have any.
We look at their squad and the only ones capable of still delivering big results are Cav (on sprints), Lutsenko (stage races and the odd classics) and Moscon (classics, if he ever returns to his Ineos days). Neither one of them has any good support whatsoever, so their chances don't diminish by the team going on a particular direction. And right now, for a broke team like Astana, maybe a sprint train is cheaper than getting good mountain domestiques for Lutsenko, top classics guys to support Moscon or new leaders. Not to speak that good leadout riders can often be also of good value during the spring classics.