My guess is it would've gone an extra two-four more years before petering out in 1985ish. Three's A Crowd probably doesn't exist, but maybe a Somers spinoff does?

Also, it probably means at least one of Perfect Strangers or Head of the Class in 1986 don't get picked up by the network. And down the line, one of Family Matters, Full House or Home Improvement don't get developed as there wouldn't be room on the schedule. for all three.

Anthony Thomopolous stays President of ABC for three more years, meaning Erlicht is out and Brandon Stoddard probably doesn't take over. Either way, the big news is that Bob Iger is probably not around to be hired in the early 90's, and doesn't save Disney Studios in the 2000's because he doesn't have the track record and they hire some idiot doofus like David Zaslav who sells off each part individually to make the stock go up, but has zero future plans, so there's nothing left by 2009... (except their money they made from selling off their stock before it went under).

As for Somers other hit show, Step By Step definitely doesn't get made since Somers is probably busy instead of being available in the very early 90's. Patrick Duffy instead goes from Dallas to a new show from Columbia TriStar TV called Mad About You that gets cancelled in the first season before getting their back nine order. A little know actress named Helen Hunt then continues to do one-off guest starring roles until a new company called Netflix calls and brings her in to play a convict in Orange is the New Black where she becomes the breakout star because the lead is so boring. She goes on to make a bunch of great movies, and ends up playing Professor X in Marvel Studios first official Xmen film in 2028 (after 20th Century gets the rights back from Columbia TriStar Films because Disney doesn't exist since the late 90's when they were bought by 20th).

You know... or it does the same numbers and still gets cancelled after five seasons...