Because in the long run, energy costs will account for the majority of mining costs, and only when the benefits are greater than the costs will the miners work and the network will be safe. As the block reward is halved, if the price remains unchanged, the miners' income will also be halved. At this time, some mining machines may have to shut down. Bitcoin is unlikely to experience a price surge after every halving. Once the block reward reaches a certain level, it may pose a threat to the security of the entire network. To avoid this, we might have to set a minimum block reward for miners, like Monero does, but I don't think that solves the problem.
That's why I started Black PoS, but got a bigger picture since then.
I also think POS is far less secure than POW, and POS greatly weakens decentralization, which is the core feature of the blockchain.
PoW and PoC (capacity) give more time for decentralization, however the end game is quite similar: eventually, all matter-energy is owned, so one can't join a consensus without a permission like in PoS.
Could using non-computing devices solve this problem? Such as storage devices?
Why do you think the matter consumption is a lesser problem than the energy consumption, afraid of Universe's thermal death?