They wouldn’t need a dongle for phone cases, they could easily make room for a headphone jack, but they don’t as a concious decision for a few reasons:
“people don’t want or use them” as “the market” has “spoken”. In other words, phone manufacturers removed it, the public didn’t revolt, people bought more wireless headphones and other manufacturers followed suit.
less lint in your phone I guess. A few more extra phone features, or capacity or battery life. USB-C truly is the future. Why add a feature that most people won’t use instead of features they will use?
I mean there are many MANY reasons why we would want to keep the jack:
we could actually use the FM receiver that phones are capable of and use the headphone jack as an antenna, as was the case when smart phones started hitting the market.
batteries die and the wireless headphones have batteries. Really kinda silly to have two products that can do what you need with one wire but they no longer can because there are no magic pixies to send over the air from one to the other.