Except that the removal of data from tables is fairly expensive process - you have to delete the data, which means running queries (which have to scan the data,

Except that the removal of data from tables is fairly expensive process - you have to delete the data, which means running queries (which have to scan the data, write a lot of WAL and modified blocks, etc) and then do cleanup (which means vacuum on the table and also on all the indexes). That's expensive. With well designed partitioning scheme you can simply drop a partition, and you're done. Much faster/cheaper. But this is just one one benefit of partitioning - I've mentioned the possible benefits for planning elsewhere in this thread.