That is kind of the issue. They have oversaturated the Robin identity where now they don't have room for all the Robin-like characters and it is this role where characters "graduate" from. I'm not the biggest fan of it turning into that.
Back in the 90s Tim was the bigger character than Nightwing as well. That continued all the way up until arguably when Dick became Batman under Morrison. Tim as Robin was the one that set the ground work for characters like Nightwing and Batgirl getting their own solo titles. He was the first Batman character to expand beyond what the typical role of Robin was. The idea would be if Dick was Robin back then, and even if he was deaged, he would have probably occupied that role and would have gotten the large push in the 90s. So instead of Nightwing it might be a Robin solo book that would still be ongoing right now.