As far as I can tell, there's no real reason it has to stay deflationary.
At some point there could be a consensus that inflation would be good for bitcoin and they could patch the client to start increasing the new bitcoins per block.
tbh I don't know why it isn't inflationary, if it was you could remove/reduce transaction fees as miners would always have an incentive
In theory there could.
In practice, the vast majority of the bitcoin users are speculators, and will fight tooth-and-nail against changing away from being deflationary.
If the miners think that they'll get more value out of the system by creating new coins forever, they'll make the change. They have to be careful not to destroy confidence in the system; this will be part of their value assessment.
Which IMHO is what will kill it; it's just a terrible feature for a currency.
That bitcoins are divisible completely misses the point that prices go up faster than they go down, i.e. they are sticky, and deflation is painful and feeds back into itself leading to a recession. Divisibility is not the correct solution, inflation is the correction solution, i.e. add more money rather than expect all prices to go down to adjust to the supply of a fixed currency.
However, it's not in the interest of users of BTC to use a wildly unstable currency. My prediction is that bitcoin will be replaced at some point by another crypto currency which fixes some of its major flaws - particularly its deflationary nature.