This doesn't even get into the dongle problem. The simplest example is that if you want to hook up Apple's flagship phone up to their flagship laptop for development, you need a dongle. And if you want to share one of their flagship wired headphones between the two devices, you need another dongle. Therefore if you want to connect three of Apple's flagship devices together on a daily basis, you can't go anywhere with…
All those arguments were once said when the first iMac came out and all it had were USB ports. All devices will have USB-C connectors soon, and instead of getting dongles, you'll be simply replacing your cables to be USB-C versions of them all.
It seems that USB-C will be more successful than Thunderbolt 1/2 have been, but Apple really isn't helping it with the way they're doing the switch (some devices USB-C only, some devices USB-A only, Lightning headphones...).