That is a really good point to ask and even if not 90% and Bitcoin has 70% dominance like Coinmarketcap says, why do we think it is worth it to keep hunting for new altcoins?
I guess the answer is that if somehow your new altcoin can even gain 0.1% of dominance in future, that is a lot of money. For me, if all my basket or Monero, Waves, Litecoin etc all make 5% together,,, I am happy!
Makes sense to invest in the altcoins that have a chance to obtain a small % of the total crypto marketcap. Instead of trading the altcoins, having a proper long term altcoin portfolio can be a good clarification for the questions asked by the opponent. Predicting the market cap of altcoins can turn to the gameplay, the end depends on the skills and expertise of the player.
New price momentum can hit the market less than 2 weeks which is the total required time for last Bullrun. Decreased Bitcoin dominance will open the new gate for altcoins that have chosen by the majority of the traders. Preferred altcoins share the common characteristics like the legit project, strong background team, raised funds, real advantages over competitor projects. These shared attributes are the main points why investors prefer to invest in before-mentioned projects than putting all money in the dominant based cryptos.