After a hard fork, as long as both branches survive, there are twice as many bitcoins around. If you had 100 BTC before the fork, then after the fork you have 100 "Series A" BTC and 100 "Series B" BTC.
Do you really not understand that when you say 'series A bitcoin' and 'series B bitcoin', you are really saying 'bitcoin', and 'some altcoin'?
It's true that you will have a free supply of 100 units of the alt coin, which may or may not retain value over time, depending on whether anyone continues to mine it - but you will still only have 100 bitcoins - whichever branch eventually establishes itself as the 'real' bitcoin.
Do you really not understand that either series can claim to be the real bitcoin, and that either or both could survive, depending only on political factors?
At the last hard fork, everybody agreed that "Series B" was "the real bitcoin" and "Series A" should be excommunicated and left to die, even though "Series A" was "the real bitcoin" for a while after the fork point.
If a kid creates a hard fork on his laptop, almost certainly every other bitcoiner will consider "Series B" an insignificant altcoin, and continue viewing "Series A" as "the real bitcoin".
If Gavin does a hard fork to increase the block size, my bet would be that "Series B" will again be accepted as "the real bitcoin" and "Series A" will quickly die out. But I do not think that this outcome is totally certain; if Gavin bungles the politics, conceivably "Series B" could be rejected by the community, or there might even be a schism, with Sunnis and Shiites both claiming to have "the real bitcoin".
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