Unfortunately they won't be replaced one-for-one. Some routes offer easy up-gauging, but those frequent-fliers who are going to "love" the removal of 50 seaters will hate when *mid-sized-city* has three flight options then sits four hours in Denver for the next Podunk Express. Hey my nipples are as hard as the next guys to move flying to mainline but the landscape of air travel we and the public have grown accustomed to will be changed. Frequency breeds business, always has.
Commuting could get even worse too. It's not all gold.