I like your points mythical. Has anyone ever tried making lightweight hollow cranks out of titanium? That could help to reduce the fatigue issues of aluminium. I assume engineering hollow titanium cranks would be a major undertaking though. Sorry for the rambling.
Yes, back in the 90's a number of companies made welded tubular cranks from titanium. Sweet Wings made a few titanium prototypes but founder Craig once told me that all were lost. Morati, a Czech aerospace company, had theirs in production but those cranks ultimately proved inadequate due to many customers experiencing failures.
Then there are Breezer IRD titanium cranks, Cook Quality Products and a bunch of others that all have disappeared into history. The most notorious of them all are the extremely lightweight Fly Sports Propellor cranks. Sadly, these cranks weren't engineered to last.
The most recent iteration of titanium cranks I know of aren't light and made in China.
Most of the problems with titanium cranks stemmed either from the engineering or were to do with welding and residual stresses within the crank arms, or both.
Back in 2007, I design my Myth crankset design with Reynolds 953 arms. We tried to make a titanium version with 6Al/4V tubes made by Litespeed (because Reynolds no longer made their seamless titanium tubing) but the tubes cracked at the seam, so that plan fell into the water. Currently, I'm developing a 3D-printed titanium crankset. This eliminates welding and makes for a lighter design with more freedom in terms of shape. We just finished the first prototype and proceed with testing.
And TI is significantly heavier than AL.
Yes, in terms of mass, but a titanium alloy makes for lighter crank arms with better stiffness and strength than an aluminum alloy because less of the material is needed to do the same job.