We're nearing the 50% mark (~53% as of now).
Irrelevant, really.
Most of altcoins are definitively in a bubble and their contribution to the altcoin "market cap share" won't last for long. Even the smallest clone/scamcoins have a market cap of millions or tens of millions now. That is not sustainable in any way - if you consider that half a year ago a 10 million marketcap was enough for the top 20 on Coinmarketcap.
Not only are the alts in a massive bubble, a great deal of the market caps are fake. Ripple and NEM are premined trash; a huge percentage of their supply is intentionally withheld from the market by those who own it (I'm not talking about "holders" or similar, but the people who created it and took the premine). In reality, for example, Ripple's market cap should be calculated (if we use the standard way of calculating it): ~8 Billion x [currentPrice] and not ~38 Billion x [currentPrice.
But some of the main altcoins (mostly ETH and maybe also LTC and Monero/Zcash and maybe a PoS coin) can definitively challenge Bitcoin. ETH is working on own scaling solutions (sharding and Raiden). In the case of LTC.
ETH - Centralized FED 2.0 coin with no real use cases besides companies building up their own projects.
LTC - Silver to the Gold.
Monero - Small niche market with real features (anonymity, fungbility, privacy).
Zcash - centralized scam coin (see founders reward & master keys).
ETH scales much worse than Bitcoin, and so do Monero and Zcash. If they do sharding, Bitcoin will do sharding just several times more efficiently.
It bounces back from 50% to 57%. Looks like it will go back to 90% very soon. It is just someone manipulating the market.
This observation is random and your numbers don't mean anything.