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.
I think institutional buyers prefer eth. Also when it comes to selecting blockchain, Ethereum is the preferred one by many established business. Recent Visa moves is manifestation of ethereum's popularity and it's wider acceptance.
No doubt Polkadot is also a good option but it has a long way before it catches you with ethereum.
Eth 2 will further accelerate ethereum adoption.