Cant say that boxing is loosing popularity and MMA is gaining it. Each sports has its own fanbase. But the amount boxers receive after the fight was always a gem that attract other fighters to switch to it. People announced their desire to switch for long time, but I think Ngannou made it most laud. Now, like bittraffic mentioned, new UFC star starts to pay more attention to boxing. That isnt healthy for UFC. Dont know how they gonna react. Still continue to bind their fighters with contracts, or let them have a boxing fight if that wont harm their UFC career.
Unlike Francis Ngannou, I think others are free to switch whenever they are given good offers to crossover and this is not a problem for Dana because, at the same time, his fighters will gain more popularity and bring that to MMA which will gonna be a good thing for them both. They cannot be blamed because the huge price gained in boxing while the fight is in the Middle East can really change their life and make them more famous than they were just on MMA platforms.