Here's one easy rebuttal: on the Sia network, renters are able to choose which hosts their store data on, and they will use a number of factors to decide which hosts to use, not just price. For example, low latency is important for some applications (e.g. a CDN), so it's not acceptable for host in China to be serving content to users in Europe.

Another way of thinking about this is to realize that the centralization of all data storage in China has not happened already. Why not? A motivated company (one as big as Amazon or Google) could move their datacenters to China if price was the only consideration. But they haven't, because latency (among other things) matters.

I'm not sure if this rebuttal applies in the case of Filecoin, since it's not clear to me to what degree renters are able to choose the hosts that store their data.