As for altcoins, the exact opposite is true: Bitcoin is on the verge becoming the minority of market value for all digital currencies. Non-bitcoin digital currencies are bigger than ever, and getting bigger.
Bitcoin alone is 59% of the crypto market cap with a $1.2 trillion market cap and that number is expected to rise when Bitcoin hits a new ATH sometime next year, so no, I can't entirely agree that "Bitcoin is on the verge of becoming the minority in market value".
Altcoins are many are are constantly increasing with more investors, but a lot of these investors still invest in bitcoin too so it would be difficult to see only altcoins rising while bitcoin is not.
These altcoins have an unlimited supply and this affects their price a lot so it will be difficult for their market cap to increase significantly. A lot of them have a high market cap simply because of the billions of coins in circulation. I don't think any single altcoin has been able to outperform bitcoin in the long term, they mostly prosper short term.