Maybe, maybe not. BTC is the benchmark against which all other crypto gets compared but is too slow and expensive to do everyday transactions on. PoW in general is too slow and power-hungry for high-volume transactions and ETH going PoS will make it the likely defacto coin for everyday stuff. There won't be anywhere as much of a need for alt-coins once that happens.
The main purpose of pow isn't exactly for transactions, it's to mine new coins. Right now, most of the computing power is for mining new eth rather than transactions.
Even when eth goes pos, there will be other new coins to mine. And no, eth will not become defacto coin. Eth now is used mainly for ERC20 transfers. No doubt it's the main method of transfer. But there are other methods using ftm/binance network..
Anyone outside eth doesn't need erc20. Cross chain transfers doesn't need erc20 too.