It all depends on the market, which is why there are some Classic Hits stations that are either Pop leaning or more balanced between Pop/R&B and Rock that do well in other markets, such as WCBS-FM in New York. The reasoning for WLS-FM's Rock lean is spot on, and it brought them huge initial success, so one has to wonder what's going on that's causing the station to be underperforming now.
Cumulus seems to follow the WLS-FM playlist template with a lot of its smaller market classic hits stations these days. In SC, Cumulus's WOMG in Columbia and WIWF in Charleston both follow the rock leaning playlist but will also toss in Madonna, Tina Turner, Prince, Michael Jackson, Cyndi Lauper, Bee-Gees, The Go-Go's, etc, and some select stuff like "Play That Funky Music", "September" and "Superstition" from the 70s that wasn't rock. In FL, Cumulus's WGLF in Tallahassee and WJTQ in Pensacola are the same.
Cumulus's WJJK in Indy follows the above pattern as well and is one of the most successful classic hits stations ratings-wise in the country.
Conversely, Cumulus's KCMO in Kansas City, WHTT in Buffalo and WGRR in Cincinnati - all heritage, extremely successful classic hits stations - are slightly biased toward rock, but still play a lot of late 70s and 80s pop you aren't going to find on WLS-FM like Paula Abdul, Culture Club, Chicago, Whitney, etc. I believe these stations also use jingles, unlike a lot of Cumulus classic hits stations. WGRR, WHTT and KCMO both have classic rock sister stations, so they are probably trying to stay out of their way as well.
I really think it just comes down to market dynamics. If WLS-FM's in-demo numbers start to fall or continue to fall, I expect they will adjust the playlist back. It's just a bit tougher with Cumulus and a lot of formats in major markets because there aren't other similarly sized markets to compare them to like with Audacy and iHeart. Classic Hits stations in 30-something sized markets and 80 and smaller sized markets are going to be different from WLS-FM.