I'm not so sure of that. I believe that the dominance metric is taken way out of context---people first claim it's useless when Bitcoin's dominance is low, then when it's going up they start celebrating and trash altcoins.
I know a lot of people want to see altcoins vanish from the market, but that's not going to happen, regardless of how little utility they bring to the table---utility people never cared about with how they only focus on massive pumps.
If you ask why people trade altcoins, in most cases they will tell you to increase their satoshi count. The way altcoins pump and dump makes it very lucrative for traders profit. All they need is a chart. Utility is meaningless.
That's certainly true, mate. Not every altcoin will vanish from the market, as there are a select few altcoins which truly deserved to be mentioned. This is attributed towards their active development, innovation, and most of all, real use cases within the mainstream world. Ethereum is an altcoin which fits the bill nicely. Since Bitcoin lacks smart contract capabilities, Ethereum fills the gap whenever it's necessary. Other altcoins like Dogecoin and perhaps Litecoin, serves as speculative instruments for traders. They allow anyone to earn money easily as they're much more volatile than Bitcoin itself.
As for BTC's market dominance, it greatly depends on how altcoins' prices are within the market. If most altcoins are in a "bloodbath" while Bitcoin is on "the green side", then rest assured that BTC's market dominance will be at an all-time-high. The same thing happens vice versa. One thing for sure, is that Bitcoin's dominance will always be higher than that of altcoins no matter what. This is true because Bitcoin has been able to remain as the world's largest cryptocurrency by market cap for a long time. The only way for Bitcoin's market dominance to go to zero, is if another cryptocurrency takes its place on the market. But we all know that this is very unlikely to happen within the future.
Nonetheless, I believe that investors will continue to find Bitcoin as a more appealing option than altcoins themselves for a very good reason. The stability and security of the pioneer cryptocurrency has made the headlines within the mainstream world. Since it's the first cryptocurrency it comes to attention to any newcomer into crypto, it's no wonder why it's extremely popular nowadays. Just my thoughts