Is it considered polytheism (shirk) to believe that refraining from buying a motorcycle increases one's lifespan, despite believing that lifespans are predetermined, while also fearing to buy it due to the high number of accidents? And does this apply to the Almighty's saying, "And do not throw yourselves with your own hands into destruction" and "And do not kill yourselves"? What should one do when in need of one?

Death and one's appointed time (Ajal) are decreed by divine will and destiny, inscribed in the Preserved Tablet, and cannot be altered. However, this does not negate the principle of causality. Humans are required to employ means and take precautions. One should not exaggerate caution and meticulousness to the point of obsessive doubt (waswasah), nor should one be negligent in taking measures for safety, while believing that the matter rests solely with Allah. An example of this is the questioner's fear of buying a motorcycle if it is highly probable that it is more prone to danger or due to their lack of skill. This falls under taking permissible precautionary measures. As for fear without an apparent reason, it may be a pathological fear that requires treatment, while keeping in mind faith in Allah's decree.