Hopefully that will happen, polkadot will continue to grow and be able to compete in the market and dominate at a good price. but we need to know together that Ethereum will not be defeated by any Altcoin. Ethereum will still be the 2nd digital currency after Bitcoin
Everything is possible in crypto though, ETH surely have massive market capital as of now even far surpasses those coins that are below #2 but isn't it good if there happens to be competition between ETH and other coin that could compete while having quite close market cap, if that happens we definitely won't see eth blockchain being too overflowed with all the transactions and the upcoming developers who wants to create token some of them could go to ETH's competitior.