1) Well even if you are right and the system can forget about old transactions, it still is broadcasting all current transactions to all nodes which doesn't scale. And nodes will need to keep collecting blocks forever which also doesn't scale.

2) Yes I know it is p2p that is why you have to worry about how the network stays connected and not just assume everyone will be connected. There are issues like islanding that you have to worry about if you remove a central point like the irc channel is now. This issue is more minor because this problem has been solved before and you don't have to change the way the currency system works to fix it later. I only mention it because I think the solution to 1 will also help here.

3) I know the number is really 21million*10^8 or whatever. But that is still fixed and the size of the bitcoin economy (hopefully) isn't fixed. The idea is that it will grow right? So that means that the value of bitcoins will have to change over time which isn't ideal. A currency that has 0 inflation/deflation is the most ideal right?