In the early years there weren't so many alts like they are now so it's dominance going down whenever there is a bull run and going up when there is a bear market is normal.
Basing on the recent movements, i would say it's going down and how further it will go will depend on how long we shall experience the bull run.
Below is a graph of Bitcoin dominance for the past 5 years. Take not of that massive deep during the 2017 rally.
Its market value have shown a downtrend for the previous years but I doubt that its dominance is falling. For the past years the price of Bitcoin fell but same as with other cryptos' market value. From this, there is a parallelism with regards to the dominance, meaning, Bitcoin is still on top of other cryptos whilst the downfall of the market prices.
This metrics only show the percentage of 'how many' Bitcoin trades are executed on the market compared to the whole volume. Simply put, it shows whether people are trading more Bitcoin or not. It doesn't really tell 'how well' Bitcoin is, and is only useful to find an entry point for the 'alts party'.
Don't be too concerned about the numbers.
Numbers are more of a statistical proof but as what being said, the number of trades fell down but that is due to the circumstances. People did not choose to trade at times of downfall simply because they chose to hold at that point and to patiently wait for the uprise to occur.
I think bitcoin will keep dominance for long time, and will have more money than other altcoins and is not the time for altcoin bull run now.
This would only happen, referring to alts uprise, if ever Bitcoin will face a certain issue that would make its wide range of users, uninterested towards Bitcoin. So it would really seem to be constant, that Bitcoin will remain dominant, given that its reputation is still fine.