A 2-year horizon -- even for massive, strategically sound projects -- is a non-starter, simply untenable in virtually every situation I've encountered in my 21-yr career in software dev and mgmt and consulting. Not saying you're wrong to think and advise in those terms, just that the opportunity to do so is exceedingly rare. Big public companies tend to be overly constrained by quarterly myopia, and startups usually don't have runway to attempt to plan that far out. Personally, I once endured a project being shelved after 15+ months of solid, productive effort. I've also been the catalyst, at least twice, for adoption of paradigm shifts that ultimately spanned ~2yr timeframes (one was performance as a first-class citizen, and another was incorporating RWD (responsive web design) into a large company's MO. These successful cross-cutting / interdepartmental changes took time, but I don't think either of them could have happened if they'd ever been pitched up front as multi-year projects. Maybe you've just been luckier finding far-sighted decision-makers.
(If I seem bitter, I'm not. Just sharing my lived experience.)
I think it depends on scale and the inertia involved, and my original point was that accurate estimation is possible.
There are many other challenges as you point out.
I am currently on year two of a five year project. The key (and part of the essential difficulty) is finding quarterly or six month valuable deliverables that help ease anxiety of everyone involved.
It is not easy, but it is possible.
While all this is going on, I'm also working on a design document for a potential 10 year project. I doubt that I will pull this one off since I'm having difficulty finding those quarterly deliverables, but I'll keep grinding away.
(If I seem bitter, I'm not. Just sharing my lived experience.)