I agree that what makes a bike fit for purpose is very subjective but the basic idea of an adventure bike or overland travel bike was established years ago and models such as the BMW R100GS and Africa Twin pretty much hit the nail on the head. Sadly, in my opinion, these have evolved into what I now refer to as the Starbucks tourers whilst the manufacturers have been developing smaller capacity engines which are much more suited to long distance travel (BMW XC and KTM 690 for example) as well as being a good basis for a relatively lightweight off road capable machine. Whilst they've made a reasonable attempt at a trail/enduro bike using these engines they've not come up with a Tenere equivalent. Is this because the market is too small - I don't think so as -with the Starbucks model a lot of purchasers are buying into a style or image with no real intention of using the bike for an overalnd trip. This has a downside in that the manufacturer will fit cheap parts to keep the retail price down but at least we might have something that would form a good starting point for a travel bike without having to invest 25% of the cost of the bike just to increase it's fuel capacity.

I think the real reason that we're not seeing these bikes is, as has been said in the car industry for decades - small car (bike), small profit.