Even if you make usable size 25M, there's still more tx than space and people will "fight" for that space through higher fees offered (thus increasing the higher average). Because, math.
Ah OK, if you part from the assumption that 2+x MB won't be enough for the current transaction volume, then I can agree.
But I don't think so. The present enormous mempool sizes are created because of the backlogs of previous hours and days.
This chart (transactions added to the mempool per seconds) confirms that Segwit for now will be enough even without the 2MB expansion. Average transaction rate (mempool, not blocks!) is about 3,5 per second now - if we think of an "effective" capacity increase to 2,2 MB by Segwit alone that will mean that the blocks - giving today's transaction rate - will be only full to 55-60%, given the current
average transaction size of about 500 bytes.
On blockchain.info you can also find charts that indicate that the transaction rate grows less than 50% per year. That would mean that with Segwit alone blocks will be full at the end of 2018, and with Segwit2x you have time until 2020/21. But you have to take into account that Lightning Network and other approaches will very probably mean that the on-chain volume will grow slower.