Even if hey left, with 200 member countries it wouldnt amount to much of anything. UEFA(Europe's Governing Body) may oppose Blatter, but this guy is a better politician than most politician and the fact that Football is next to religion means he can afford to be as opaque as he can get away with, because how the federations are structured from the grassroots allow for very little oversight.

If a major football country like Germany or Spain left FIFA the FIFA World Cup would lose one of its strongest competitors and it wouldn't be a World Cup- FIFA would fall apart like a house of cards. Blatter would have to step down because of popular pressure on him (my humble opinion). The chance of a national association 'seceding' from FIFA is very, very unlikely though. I agree with your assesment of Blatter's shrewdness.

Spain is strong as a Club League, not so much as a National Team. Germany is important to the World Cup, but it's not enough... Remember that in every World Cup, some "important" countries are left out in the Qualifiers.

To really hit the tournament, you would need the support from several historical and relevant members in each Association. As an example: USA, Mexico in Concacaf; Argentina and Brasil in Conmebol; Italy, Germany, France and England, Spain (mainly for their leagues) in UEFA.

You make an interesting point but:

Let's say Germany 'secedes' from FIFA. I know for a fact that if Holland (I'm Dutch, but same thing goes for England) were to win the next World Cup in the absence of Germany, it wouldn't be taken seriously. I'd say most Dutch football fans would rather not win it at all in the absence of Germany. If they didn't qualify, well that's a whole different story, then they obviously weren't good enough to start with.