You also need to remember that there is no "set" fee for a TX fee, but rather the fee is based on how big the TX is (among other things). Also there are many "free" TX that are confirmed in every block (or almost every block)
You are describing the current policy that is implemented in the reference code. There is nothing preventing miners from adopting other fee structures that will increase their revenue, and there is nothing preventing users from adopting other fee structures that will lower their costs. In the end, economics will determine the fees, and not the policies of the Bitcoin Foundation.
Even if the block space is available there is noting that says each block must be "filled" to the max block size. Even if the max block size is large, miners to have a small incremental cost for including each additional TX in their found block so they would likely not wish to include a lot of no fee TXs