If you delete your data map then effectively you have shredded it forever. Without the datamap the chunks are just random bits of data to anyone looking at them.
The reason is that without the data map there is no way to know what chunks out there were part of any file.
Then additionally without the order of the chunks in the file then there is no way to decrypt the file as you need to have the chunks in the right order.
While not 100.000000% perfect the larger the network becomes the exponentially harder it becomes to look at every single chunk and reorder them trying to decrypt them. Additionally there is no way to know who they belonged too. Its really hard with 2^128 (or 2^256) addresses for chunks, if some angency tested every chunk address for actual chunks then the solar system would be dead by the time they got through them all, so you see its really hard to try and just find chunks to examine let alone try all the combinations of order to try and decrypt them.