I already explained how this can be solved. If there is a liquid exchange between the altcoin and Bitcoin, then the altcoin is just as liquid and usable as Bitcoin in terms of transactions.
But the problem is if everyone wants to sell the altcoin, i.e. no reason to hold it, then there is no such liquid exchange.
Friction always imposes costs. No how matter how good you make your exchange, it will always be more expensive than not needing to make an exchange. A line is the shortest distance between two points.
It is possible that an altcoin could have strong anonymity that is very important to smaller percentage of the market, and that would be a reason to hold it.
There might be some other features that are important enough that some percentage of the market must hold the coin and can't hold Bitcoin.
Strong anonymity is something that if you need it, you need it, and not having it isn't acceptable.
Can you deny this logically?
Any other such features?
Any feature that is so heavily demanded will be added to Bitcoin, because that is the most economically valuable solution.