The 42,000,000 bitcoin thing isn’t like altcoins at all, as it introduces no inflation. It’s simply like saying, “From now on there are 24 inches in a foot instead of 12.” It doesn’t make anyone any taller. Everyone’s stake in the ledger remains the same, because for every bitcoin they had before the fork, they have two after the fork.
The cleanest way to see this is to realize that “1 BTC” is really just a shorthand for a certain percentage of the current ledger (and a 21,000,000th of the total final ledger). Say for you it is 0.00001%. That percentage is invariant under forking. That is, it always stays the same no matter how many times you fork Bitcoin with its current ledger distribution. Bitcoin could fork into four chains with 84,000,000 total BTC, but you would still have 0.00001% of the total, and your purchasing power would be unaffected.
Altcoins do dilute Bitcoin’s value, though only a tiny bit so far. Forks that result in persistent splits do not. Anyone who thinks they do is merely committing a mathematical error, exactly the same as the who thinks he’s taller because the half-inch has been renamed an “inch.” Percentage of the total ledger is all that matters.