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.
Altcoins do not
individually have an unlimited supply, and people buy one altcoin at a time, not the whole market.
Also, the number of units a coin has does not relate to market cap: that's not how math works

.
I didn't say that there will be another digital currency that will surpass Bitcoin in market cap--although I don't think that's impossible--I merely said that I expect Bitcoin's overall marketshare to dip below 50% at some point and go further down after that.
I believe the overall market for digital currencies will be 10x what it is today in ten years, but a majority of that growth will be non-Bitcoin (and non-blockchain, imho) currencies.