As noted to a degree above why does there need to be a defined 'good', 'right' point of market dominance for Bitcoin. That is something for the market to determine and plays both off what other coins have to offer and what bitcoin has to offer. Bitcoin is still has the biggest share as it is the household name, with a strong support, following and direction, but that doesnt mean it needs to hold 50% of the market. It can do its thing at any % (though I guess it may lose some of its shiny appeal if it is 2nd on CMC).

Think about where Ethereum was in January last year. I'm sure most people didn't see the huge potential and the massive crypto explosion (token-wise) that would be caused as a result of the eth network. So at the start of last year there was a bigger market dominance by bitcoin because the other players in the game were weaker, but that dint stop eth from executing its plan and becoming a star player that has earned its slice of the market.

When I got into Bitcoin it was the easiest thing to buy with $$ and therefore was required in order to purchase eth or other coins, some of which may or may not be shit (im sure future me will give me a lecture in time). This added to its utility and thus gave it a bump in market dominance. If you go on any major exchange you can now pick up around 5-10 crypto assets which has eroded the 'gateway' effect that bitcoin had.

I don't think there is a perfect, right or good market slice for bitcoin to hold on to (nor do I see a need for there to be one), but I see it being the big boy on the block for some time.