Imo it is the only way for this tank not to leak value and thus be a worthy place to store value at all.
Also, as the blockchain grows longer, the less incentive competing cryptopcurrencies have to start their own.
People will obviously be drawn to the network most resilient against attack.
Is a longer blockchain itself is attack-proof, even the total computing power is dropping?
No. Well, it's less susceptible to be written over from the beginning, but that doesn't matter. Any real attack will work off of a recent block so current power is all that matters.
This is why clones are silly. Unless something way better comes out (and then we'd all rationally port to it) it makes sense (from an individual and group point of view) for everyone to join the strongest network.
