I'm not sure that we sold Coman in 2015 in order to fund Douglas Costa's transfer in 2017.
We sold Coman after Pirlo, Tevez, Llorente and Vidal left and after we bought Dybala, Mandzukic, Cuadrado, Alex Sandro and Khedira. We already had an excellent team and Coman was aware that he won't get as many chances as he wished. We probably weren't desperate to sell him, but we weren't gonna stop him if he wanted to leave. That was it. He had a fine career, but there were no losers here. We won the following 5 scudetti and made it to a CL final. Imo, he wouldn't have helped us achieve more, we couldn't know that a banter era will follow six years later, and Coman probably wasn't prepared to be a bench warmer for five years and then be a starter in 2021.
I'd even say that the sentence: "We sold Huijsen so we could buy Kelly" isn't true, but this happened in the same season so it's normal to make the parallel.