IMO the market dominance does't really matter as long the project delivers in long term, investors look at Bitcoin differently, a much superior assets compared to existing ones with high volatility. Many alts have different purpose and never would be as popular as bitcoin, if bitcoin moons insanely high of course people would sell their pairs to join the ride. Only good alts could co-exist with Bitcoin and grow together, thats why i hold both bitcoin and alts to avoid scenario where it bleed out all your potential gains