Says who? It has been used before.
Steel axles are used in most throws, and titanium isn’t really stronger than steel, just has a higher strength to weight ratio. The Luftverk Evora Ti7075 uses a titanium axle, and I believe some Yoyorecreation throws have as well. If you don’t strip out your yoyos with steel axles, you’ll be fine with titanium. Only reason I’d want it is because it is my “dream design”, so it’s “Ultra Premium”, and would save a very minuscule amount of center weight.