Give your customers an inch, and they'll start demanding a mile, eh?
If you're going to complain that eight is too few mercenary slots, why even put another number on the limit of mercenaries? Why do you suppose that it wasn't just made to be limitless in the first place?
Anyways, there is very little besides any twinge of convenience that one might get from having all of these extra heroes lying around to merit the existence of more mercenary slots. It's not like it's that taxing to salvage off runes and insignia, replace the mercenary, and then re-equip them. Perhaps an extra storage pane or two would suit you to hold the upgrades that you aren't using when you have to switch them around.