From what I am seeing miners should be able to spend their proceeds in 6 blocks not 100, so there must be justification for the 100 block wait on mining.
In the event of a fork, all of the non-conflicting transactions will eventually get into a block, regardless of which fork. Except for the generate transactions, and transactions that spend them, etc. 100 appears to have been picked as a large round number that would prevent orphan transactions even in the event of a very major split.
Lowering it would cause a hard fork, but raising it would merely require that miners upgrade.