You have to remember there are rules. Whether you like it or not, and whether this is fair or not is irrelevant, visas are not a right, they are a privilege. Each country (or here, union of countries, the Schengen Area) is free to decide who can enter, and under what conditions.
The rules are clear. You cannot ask for a visa to travel to country X when you intend to travel to country Y instead. Especially for a long duration stay like the one you are discussing.
Also remember that even if the probability of you getting caught is probably small, the consequences if you are caught can be devastating. Being turned back at the border, spending hours or days is “administrative hold” while they decide how to send you back, and a big black mark on your record are probably not something you want to consider. And even if there are probably thousands of people every day who get through the net with “creative” interpretation of the rules, immigration officers are well trained and quite good at spotting people who try to do so. And once they have their eyes set on you, it’s usually very quick to find out the truth. You’ll lie, get caught in your lies, they will find evidence (travel plans, bookings…), and it will end badly.
Circumstances have a big effect on the whole situation. It’s a lot less likely they’ll be suspicious if you’re an affluent citizen of a relatively rich country which is just on the edge of the visa list, who has travelled many times to the Schengen Area, and spends a lot of money there, than if you’re on the edge of the poverty line (or below) with no travel history, for instance. But in the first case, you would most likely already have a multiple entry visa allowing you to travel as you wish…