Bitcoin will start losing dominance if people will suddenly feel like throwing a ton of money on every altcoin out there, like it happened in 2017. I'd say It won't happen again, because too many people got burned, with hundreds of shitcoins going to zero. Maybe ETH and some other coins will regain a bit of their past dominance, because people want to gamble with altcoins a bit, but I don't think Bitcoin will dip below 50% dominance ever again.
Got burned, get out of the market, learn from their experience and now they are smarter and won't just throw their money on every shitcoin being born today. We just have to look at the current state of altcoins, most of them even ETH are still down, while shitcoins remains to be pump and dump.
If we are going to based on the last bull run 45%-47% will likely be a good ball park number. Bitcoin price reach new all time high and altcoin market recovering. But the question is when and if people are willing to shift or throw money again in altcoin market.