Bitcoin's dominance in the market will never go away as it is the only decentralized currency with the best security. Bitcoin's dominance will only be shaken or replaced unless there is a coin with superior features to bitcoin. Otherwise, bitcoin will forever be the king of the cryptocurrency industry. As for current altcoins, they are a centralized mess, and were created for the purpose of pumping and dumping, they do not bring any practical utility or application to us. None of them are qualified to challenge bitcoin's throne.
New blockchains are offering better features than bitcoin. Some of them even have lower but faster transaction cost. They are lacking one essential elements which is decentralization. Most other blockchains are centralized or semi centralized so their price can not create much appeal for investors to put money into them. Bitcoin dominance is not content. It also goes ups and downs like bitcoins price so we can not say bitcoin dominance will never go down.
Altcoins do not lack the decentralized element, but instead they have removed the decentralized feature in exchange for speed and cheap fees. That was their choice and used them to advertise that they were better than bitcoin in every way but they failed because people need privacy, need decentralization. No one needs speed and cheap fees without being safe and controlled. That is why with thousands of altcoins created so far, there is still no altcoin that can replace bitcoin.
When the price of bitcoin falls, all altcoins will also fall, so it cannot be said that bitcoin's dominance is decreasing.