Is it possible to have Bitcoin bullrun wherein altcoin continues to trend in red, we have witnessed that altcoins has been falling from grace in last few days wherein Bitcoin is steady (apart from today's dump) and heading towards $20k to set a new landmark. I am discussing about top altcoins here wherein we should not be considering coins with low volume which is manipulated consecutively.
It's possible, when altcoin whales and investors suddenly shift to bitcoin because that is where the money is. I don't think it will be sustainable though. as far as I can remember, when we have a bull run in 2017, the dominance of bitcoin is around 40%-50%. But currently we are way above at 60%. So it's a long way to go to see this kind of scenario, it could happen though when bitcoin really goes on dramatic rise in the coming months.