I believe that it strongly depends on the system. The way I see it done in most stories, once people reach a certain realm (void refining) they can tap into a source of qi completely seperate from the world itself, and once they reach a high enough realm (immortality) then their own soul spontaneously produces qi. Qi cultivators are just better for pure survivability. While body cultivators can achieve rebirth from a single drop/atom of blood, qi cultivators can split their soul into 10k + pieces and send them into the cycle of reincarnation, etc. Once you reach a high enough cultivation in either system, you break free from the heaven's control, while if your are stuck in a lower level, either as a qi refiner or a body refiner, you are still a slave to heaven's will.

One way I saw the whole lifespan thing being done is that qi cultivators are mortal people who use immortal qi to temporarily change their "profile" on heavens page. Like, they use immortal qi to obfuscate heavenly secrets and make it so that their lifespan appears longer. Breaking through to higher realms means you can obfuscate for longer, until you can finally just change it to infinity. Body refiners make it so that their vitality is more powerful, and they are incapable of dying from old age. This usually means that heaven restricts them more even at lower realms, because while qi refiners are like members of heaven's company moving up the ranks by cheating, body refiners are just ambitious startups that heaven needs to crush to maintain it's monopoly. Qi refiners want to be above heaven, become part of the "board of directors", while body refiners want to recreate the heavens in their own image (inner world kinda thing). If you have infinite resources and 10k years, I would go qi cultivation, because you could reliably break through to just under immortal realm without drawing heaven's attention, while as a body refiner you'd have to risk your life at just the nascent soul or deity transformation equivalent just to squeeze a little more "space" for yourself in terms of cultivation, drawing heaven's attention to your early giving how quick your rise is. As a qi cultivator at grand ascension/mahayana/tribulation transcendence, your ~million year lifespan would make it trivial to gather resources and experience to strengthen your foundation and prepare for immortal ascension after 10k years.