The argument that Drivechain will replace shitcoins is not that people will sell their shitcoins or that the existing shitcoins will instantly vanish. The argument is about a change at the margin that eventually ends up killing the shitcoins or reducing them to their original insignificance.
What does "at the margin" mean? For example, when the price of the coconut drops a little in relation to bananas, does that mean that everybody will stop buying bananas and will buy only coconuts now? No. Does it mean there will be zero increase in the amount of coconuts sold? Also no. What happens is that there is a small number of people who would have preferred to buy coconuts if only they were a little less expensive but end up buying bananas instead. When the price of coconut drops these people buy coconuts and don't buy bananas.
The argument is that the same thing will happen when Drivechain is activated: there are some people today (yes, believe me) that would have preferred to work within the Bitcoin ecosystem but end up working on shitcoins. In a world with Drivechain these people would be working on the Bitcoin ecosystem, for the benefit of Bitcoin and the Bitcoiners.
Why would they prefer Bitcoin? Because Bitcoin has a bigger network-effect. When these people come, they increase Bitocin's network-effect even more, and if they don't go to the shitcoins they reduce the shitcoins' network-effect. Those changes in network-effect contribute to bringing others who were a little further from the margin and the thing compounds until the shitcoins are worthless.
Who are these people at the margin? I don't know, but they certainly exist. I would guess the Stark people are one famous example, but there are many others. In the past, examples included Roger Ver, Zooko Wilcox, Riccardo Spagni and Vitalik Buterin. And before you start screaming that these people are shitcoiners (which they are) imagine how much bigger Bitcoin could have been today if they and their entire communities (yes, I know, of awful people) were using and working for Bitcoin today. Remember that phrase about Bitcoin being for enemies?
But everything that has been invented in the altcoin world is awful, we don't need any of that!
You and me should not be the ones judging what is good and what is not for others, but both you and me and others will benefit if these things can be done in a way that increases Bitcoin network-effect and pays fees to Bitcoin miners.
Also, there is a much stronger point you may have not considered: if you believe all altcoiners are scammers that means we have only seen the things that were invented by scammers, since all honest people that had good ideas decided to not implement them as the only way to do it would be to create a scammy shitcoin. One example is Bitcoin Hivemind.
If it is possible to do these ideas without creating shitcoins we may start to see new things that are actually good.