Something I find mildly annoying about this system is that clever tactics only work on the ground floor. Cool, unique ways of setting up an Inner World are inherently abandoned the moment you Ascend, even if that method is what allowed you to be Willful enough to do so. And then you're stuck with whatever world setup the guy who lost that contest had. Sure you can dick around with Laws but unless you want to cripple yourself and start over, you're stuck with the shape and inhabitants.
Although I've put a decent emphasis on Ascension in my writeup because it is tied pretty heavily to the setting's metaphysics, it should be noted that in actually most people spend their entire lives on the World they were born in. It is a numbers game; out of everyone currently in a given World plus that World's current God, exactly one individual or merged individual can be the world's God in a year, or a decade, or a century.
So on one hand, you are right; any direct advantage you may have from cultivating your Inner World goes away once you ascend. But on the other hand, most like you will NEVER Ascend, so what you build is what you've got.
Furthermore, it is incorrect to say that you lose the entirety of your gains upon Ascension. Law Comprehension in particular is kept, as is insight into what sorts of things work - so the tools you've built and the lessons you've learnt stay with you for use on a new Inner World. Furthermore, your previous Dantian is now recursively nested in your new Dantian, and while the gains from such a thing a lesser, they are still non-negligible.
Ah, you know, I'm surprised that it doesn't seem to have come up explicitly, but how does the actual 'immortality' bit of being an Immortal play out? There doesn't seem to be any inherent life extension tied to cultivation, but I guess I'm assuming that's just a function of having a Dantian (your ageless phylactery), and eventually an Energy Body (your custom-spec disposable outer self).
Quite right. Will doesn't inherently degrade, and nor does a Dantian, which means that once you form a Dantian old age and disease aren't really issues any more.
There are of course caveats. For one thing, a Dantian alone doesn't offer any structures for drawing in or shaping Qi, so you will want to have either a biological or an Energy Body; the alternative leaves you unable to act in the Outer World. Learning to make an Energy Body isn't
hard, but it does take Qi - and if you have no Qi and no Energy Body you are pretty screwed, and it is not a good way to go. That is not to say this happens often, but it still deserves a mention. Running completely out of energy without a way to get more is bad.
The bigger caveat is that just because you don't age and can remake your body when you want doesn't mean you can't die. Someone just needs to hit you where it hurts - the Dantian. Technically, the Dantian is indestructible from the outside, but any damage to it translates directly into damage to the Will holding that Dantian's Throne - and enough Will damage will kill you.
Also, sometimes there will be Heavenly Tribulation set up to strike cultivators every 100/250/1000 years or whatnot, just to keep the ecosystem healthy. It should be noted that Heavenly Tribulation is especially effective at damaging Wills directly - and as mentioned earlier, enough Will damage kills you.
ALSO, sometimes the very world you are in dies, and takes you with it. There are ways to save a dying World, but those depend on the resources and people that that World can draw on, and sometimes it is not enough. Survivorship bias does mean that type of end isn't one doesn't consider often, but in a setting where every cultivator is a World, it is a fact that many Worlds have lifetimes which are brutish and short, and they take all their inhabitants with them.
...and now I'm bummed out about the amount of mass death in my imaginary setting.
