if a video gets downvoted down to 0$ it won't be stored.

It's not exactly true, I think @heimindanger is just using simplified language here. Once a file is put up onto IPFS, there is nothing DTube can do to remove it. Once a link is placed in the Steem blockchain, there is nothing that DTube can do to remove it.

The particular code that is used on DTube can select not to show certain videos, based on whatever logic it wants, but it's just not presenting it, the data is there and it's still fully public. All you have to do is use an alternate client (perhaps simply a modified version of DTube, depending on the license) and you'll have full access to every video every uploaded via DTube.