I believe that the market is being manipulated by whales than anything else. They're "playing" with Bitcoin, as well as, other altcoins on the market. At times, they cash out a portion of their altcoins into Bitcoin and vice versa. That's why we see a fluctuation in Bitcoin's market dominance across the crypto market. But at least, Bitcoin has maintained itself as the leader of crypto for quite some time. As more altcoins are created, you'll see Bitcoin's market dominance diminished by a small margin. The next bull run, will greatly determine how farther Bitcoin's market dominance will increase.
Nonetheless, Bitcoin's market dominance tells us nothing other than how prominent the cryptocurrency is in terms of price relative to other coins on the market. What matters is how useful the pioneer cryptocurrency is within the mainstream world. No matter what its price will be in the future, if it's useful enough for the average person then I believe that's sufficient. Things will become interesting with Bitcoin once the Lightning Network gains full speed ahead in mainstream adoption. There's a high probability that Bitcoin's price (and market dominance) will become higher over time. Which is why, early adopters of the cryptocurrency (like you and me) will greatly benefit from this in the future. Just my opinion