I personally endeavor to make docsherlock tow me and my shitty bikes out of the bush at least a few times this summer

As mentioneded by others, long answer, but many modern bikes (and vehicles) can go a long way these days.

At low miles, the list of parts that need to be replaced as part of regular maintenance is fairly short. As it ages, this list gets longer and then when you hit very high miles you end up looking at large capital items that may need to be replaced. They can keep going, but at some point the question is whether the additonal yearly investment is worth it. Most vehicles don't get to their theoretical maximum useful life, and instead are scrapped when the running costs exceed that of a newer vehicle, they degrade due to lack of use, or they are deemed unsuitable to current tastes.

On the vehicle side, I've got over 350,000 km on my 1999 Toyota 4Runner. It's getting close to end of life.

On the bike side, I've got about 40,000 km on my F800GS which is not much at all. I bought a 1200GSA that has 192,000 km, runs fine. Conversely, I have a KLR650 with about 60,000 km, which feels like it is starting to get up there but who knows, might go another 40,000 which is pretty good for a single cylinder at that price point.

Basic point is that if you are looking to get a lot of miles out of your bike, IMHO there are quite a few options. A modern bike that is commonly used for overland travel that is well maintained and ridden well will get you around the world and back again. When you get home, the miles will be high, and it'll take a fair bit of TLC whatever the make, but the memories associated with it will make it the best bike in the world, for you.