By buying something with Bitcoin, you use it as money, which is a good thing.

By buying more Bitcoin, you also increase demand after it, which is a good thing too.

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Ultimately, if there are 2 people. One who has 100% BTC and use it both as money & investment and the second who has 90% BTC/10% fiat and uses BTC as investment and fiat as money, I think the first one is more beneficial for the ecosystem.