Ethereum's proof of work algorithm was different to that of Bitcoin's, so Bitcoin ASICs could not be used to mine Ethereum and vice versa. Ethereum becoming more centralized with PoS is irrelevant to Bitcoin's hashrate.
In general, ASIC miners are only created for certain crypto assets, so if ASIC Miners are Bitcoin they can only mine Bitcoin.
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This statement isn't accurate. While it's common to see ASIC is advertised to mine specific coin, actually you can use that ASIC to mine another coin which use same mining algorithm. For example, AntMiner S9 which usually used to mine Bitcoin also can be used to mine BCH, PPC and DGB which also use SHA-256 mining algorithm.