You still need to store all transaction of all random guys. Now those transactions are split into two separate blockchains, but they occupy the same space on your HDD (in fact even little more).
And yes, there can be at most one winner. That means, it is possible, that there will be no winner at all. I.e. all free decentralised cryptocurrencies dieout. And to be honest, greed drives us now to that worst, no winner, direction.
its not a problem of hard drive space. Its a problem of limited bandwith for miners. Miners are only capable of receiving so many transactions in a 10 minute period. if the total demand for transactions exceeds the bandwidth of your average miner than people will be forced into other chains.
Anon136, you're going to run into these social and structural biases from people.
People think they can solve everything with tools XOR rhetoric. It never occurs to them you need both. Also check private message.