I respect this thought, but I disagree with the potential strength of DC Cinematic Universe and expanded X-verse on the cognitive market space and merchandising.
Marvel Studios has won any potential glut war. They have first mover's advantage. Their core family of Avengers characters have been ingratiated and loved by audiences in a shared universe for years now, doing something no one else has done. They aren't having to introduce this "shared universe" concept like Warner Bros has yet to do. As Marvel Studios continues to expand its shared universe into new areas--horror with Dr. Strange, for example--it will continue to look novel and original compared to DC slowwwwwwwwly following Marvel Studios footsteps, years behind where it should be if it just connected Nolan Bats to Snyder Supes (even with an actor change). The X-franchise is finally refreshed after about 8 years of bleh between X2 and First Class, but hardly superceding the MCU. It has its own limitations, because thematically it can't step beyond heavy handed man vs. mutants narratives. No matter how good the DC or Fox films may be, they will always be behind the MCU, and not have those benefits if cannibalization occurs and people get "bored" with comic book movies. It's DC and Fox that will suffer, not the MCU.
The only things that can harm the MCU are 1) dip in quality, which may happen but is in the MCU's control and 2) limitations on expansion. This is where trading everything mutant for the Fantastic Four makes sense, because the Fantastic Four help continue with that universe expansion. The FF provide a unique "family" concept among super-humans, human-based cosmic explorers that can organically introduce any idea MCU wants to adapt to screen, and brings Galactus and Silver Surfer with them, as Galactus can be the next "big bad" in Phase 4 or Phase 5 after Thanos is disposed of.
Not only that, but the mutant concept is basically going to be reinvented by the MCU with the Inhumans. Dumping the X-Men into the MCU post 2020 will almost be redundant, and not have the thematic impact after this big Inhumans push. It might benefit the MCU to keep the X-verse separate, as introducing the X-men after the Inhumans may be cannibalizing their own shared universe after these strategic choices.