Can I ask you to clarify something? Do you honestly believe that or is this sort of an oblique suggestion that it would be non-trivial/"unsimple" to implement?
FWIT, I have heard commentary over the years that engineering that kind of UI (where things can be dragged and dropped and moved around arbitrarily) in the way Todoist uses as its competitive advantage is diffcult to do and to get right
This is such a mission-critical thing otherwise you're like every other Todoist non-competes scrambling for Todoists crumbs. Its like literally just that to me with the infinite nesting and i can't remember if they allow swipe indenting but thats the most natural/intuitive in combination with drag and drop which can be used together and interchangably.
The issue with Todoist is they want to retain full control and deny your data portabillity and soverignty. They demand you sync thru their unencrypted (to them) cloud because they demand control or at least access to user content which they seem unwilling or unable to concede they won't leverage should buy-out talks begin.
No deal for them because of that but they still have the best UI (Sorted really is the closest thing and what I use alternatively) but theres no reason you can't swoop in and implement particularly 2 but probably all the things I've mentioned because that will be your path to folks like me showering you with money and actually out of desire for a superior self-funded project that respects and affirms our autonomy and allows for you to distinguish yourself in a non-trivial way.
Thanks
Edit: if you can cross-breed Todoist + Strongbox (KeePass) I will buy whatever you come up with provided the UI is closer to Todoist and the functionality captures both aha
Edit: infinite nesting ≈ subtasks but it can't be arbitraily limited to like 2-3 or whatever levels. If you find it philosophically problematic you could offer the user an advanced settings option that lets them toggle how many levels of abstraction/nesting levels they want to enforce and they can experiment and modify that as they settle in to your app and their own workflow