I just purchased my Kona Sutra, and I'm really happy with it. Disc brakes were important to me, as I had many scary experiences coming down hills in Washington state this year with my weight. Finding touring bikes with disc brakes, can be a challenge, but they are out there, and more and more manufactures are including them now. I also wanted to be able to pedal up the side of the wall, so I had them change out the front crank to a 44-33-22 and the back cassette to an 11-36. This seems to really help, as I'll have the weight problem for at least another year. The Kona is a heavier bike, but rides like a light bike. I'm really happy with it so far.

The bike you describe sounds very much like an Audax bike rather than an all out touring bike. They're sort of a mixture of Sportif bike and Tourer, so you get a slightly higher handlebar position and sporting handling, but still get mudguard clearance, bag and rack mounts and cantilever or disk brake mounts to cope with the added luggage... If I had to have a "one bike to do everything" I'd be looking at one of these type bikes, probably something by a custom fabricator in Ti. Back in my murky past, I was always better at longer events - 100mile TT's rather than 10's for example - so getting into doing Audax rides may well be how I while away my riding days...

* I want a bike that will last the rest of my days (I am 55, hopefully I got a lot of em left but I take NUTHIN for granted).
Steel

* I want a bike that can be fully loaded when need be, but still perform like a lightweight champ for all other occasions.

* I want a bike I can roll out my driveway on, and ride it on daytrips and overnighters, not necessarily fully loaded, across any topography. This will be mostly the type of riding I will do on it.

* I want a bike I can roll out my driveway on, and take it on a long trip, fully loaded, from my home in Southern Indiana, to any point in the continental United States. I will do this riding less frequently than day rides and overnighters, but the bike has to be capable of it.
* I want a bike that is lightweight, but strong as an ox, and I need it to be totally dialed in to my riding style and body shape.
* I want a bike that is comfortable as all get out to ride, because I intend to spend many hours at a time on it.

I'd say light weight 700c Steel Toruing Frame, possibly need to be custom made.
I'm currently building one using the Planet X Kaffenback frame, I've done wee test rides on it in various states of unbuild and I feels good, but I've not put a back rack on it yet so don't know how It's going to feel loaded.

My 26" Steel "Expedition" bike feels slow when using it for general riding which is why I went this route.
Not so sure about an "Audax" bike, these are slightly relaxed geometry race bikes designed to just meet the requirements for Audax. (That you have mudguards). Which would be fine for your overnighters, but could be horrendous after the 3rd day on it.

The Kaffenback is as near as dammit an audax spec. frame - It's pretty much what they used to refer to as a "Road Path Frame" - racing angles but a little longer stays and guards clearances - I had it's idiot brother - the "Il Pompino" and it was a cracking fixie bike, but my knees just won't cope with fixed gears anymore.

There are bikes out there that are refered to as Audax bikes, which are just a racer with guard clearances, but this is where getting to a proper custom framebuilder comes in, have it built to exactly your specification - if it's going to be the "bike for life" why not splurge a little... I still maintain that a PROPER audax bike needs to be comfortable not for 3 or 4 days of day rides - but for riding brevets of up to 1200km in one go...like the PBP - you'd have to be harder than Jens Voigt to ride that on a modern race machine :laugh:

Personally, I'd sooner have different bikes for different tasks, but a good custom Ti (or Steel - steel is good too!) do-it-all would be high on my list, had I not the space to store a few bikes...