He won't, not in the foreseeable future anyway (within next 8 years or so). Supposedly Madrid would still have Mbappe and Vinicius Junior by that time, 2 of the very best left wing forward in the world playing in the exact same position as Garnacho. Unless they are buying him as 3rd choice LWF, or if they somehow declined sharply forcing Madrid to look for replacement.
But normally speaking, by the time both Mbappe and Vinicius Junior is declining in 9 years later (say at age 32-33), Garnacho could be at his peak of power (age 28), its still possible.