Let's indulge our in the hindsight talking. If look at the team during those years, we would open can of worm on other issues. We lack a goalscorer beside Kane, either support him or back him up to get him some rest. Without that goal scorer, we had no choice but utilize Dele there. And Dele couldn't displace Wanyama and Demebele. Eriksen role is not static, and overall higher up the pitch; yet he could play deeper centrally when required. Then we had a promising Mason pre injury. Dier in midfield for a pseudo back 3 tactic when we had 2 flying full backs (a good back up for right wing with Trippier too). When we went serious with 3 at the back after Sanchez freshly joined, the midfield is stacked in quantity. Last but not least, the Winks hype from part of the fanbase. We just didn't have enough opportunity to try Dele in midfield.
Even if he developed there in due time, it's still doubtful he has become a top class midfielder. I meant we're seeing in NDombele a talented player that has been developed in his position, yet his mentality has kept him from fulfilling his potential. Or Pogba at Man U, he has all physical, technical talent, and vision, but since he can't focus on ability, he never find the consistency to showcase his talent.
I just don't see how different it may be with Dele development. He got the best route up until 2018 for his career. He didn't take his chance from then on.