The assumption has been it's a time problem, that assumption has been in place for 15 years, evidently if it's a time problem then the timeframe is a lot longer than most assumed.
The problem here isn't production or technology, it's scale and expectations, the modern 21st century world was built on unlimited access to Chinese industry, people has come to expect having power seats, ABS, phones, etc, they expect to be able to afford it and they take for granted their ability to import parts needed from China. The problem is not the ability for the west build magnet, although that is also a problem for them when it comes to high end ones, even if they develop the technology and build the plants, the size of their industrial base, which derives from population and energy production among other factors, just isn't physically big enough to meet expectations that's shaped by China.
e.g. The ability for a country the size of UK to maintain a 21st century economy without any external trade isn't really a time or technology problem, the only thing they can do is lower expectations
As for China taking advantage, humanoid robots and low altitude economy are both massively reliant on PM motors, each joint in a humanoid robot (or robot dog) is a high pole PM motor, each rotor in a eVTOL is a large PM motor, PM motors are the foundation of modern kinematics, everything that moves needs them. By the time the west start producing their own magnets, and even if they can scale up, China would already have establish an insurmountable lead. This isn't a fixed window opportunity, this is more like breaking the legs of someone in a race, yeah the leg might be able to heal, but the race would be long over by then.