The reasoning is (I'm not saying I believe this will happen just that it is an interesting thought experiment) that if people quit the shady exchanges and start trading Bitcoin ETF in substantial numbers it could begin to starve the crypto network of transaction fees and the exchanges and other crypto companies of customers.
If Bitcoin ETFs become the main way bitcoin is traded then purchasing bitcoin is just for speculation and not an 'investment in crypto and the future of finance', in that case it's much easier for the bubble to burst.