correct, but it's not (only) the unit cost but mainly the fuel costs that made most regional jet redundant.
Tendency goes towards 100+ seaters, since already 70seaters and even 100- seaters become less efficient. Workhorses are still the biggest narrow bodies like the A320 and the B737. You have the least fix costs per seat, that's why most low cost carriers use them.
But how do fuel costs per trip compare? Low cost per seat does not matter if the seats go unsold.