As a rule of thumb, the wider the zoom range the worsae the image quality. The 18-200 has the worst MTF curves of any lens Canon makes and extending the range will probably make it worse. It might come out, but I would not buy one. A pair of cheap lenses like the 18-55 and 70-300 would probably outperform an even wider rage superzoomThe whole idea of a "workhorse" lens is one that covers a very large focal length, both wide and tele, without me having to change lenses.
For my day-to-day photography, I need a focal length from 18mm to >200mm.
while that's certainly a valid way to interpret "workhorse lens", I think a lot of people would say that what makes a lens a workhorse is its ability to consistently give you results that you would be satisfied with delivering as product, or the result of your "work". trying to mush a whole bunch of functions into a single lens usually means the opposite, in which you can shoot anything, but not at a satisfactory level.
I'm not saying you're using your lens wrong. certainly, if I were just traveling on vacation and I prioritized being able to get any shot I wanted without having to carry much weight, an 18-250mm might be just the right thing. however, given the fact that you seem rather adamant about needing to go beyond 200mm, yet also being rather adamant that superzoom image quality is exactly what you want, is a little puzzling.
don't be too scared to switch lenses, even if you're in the field. exposing your camera's innards momentarily isn't quite the danger zone some folks make it out to be (unless, yes, you are in a sawdust-filled wood shop, or a decomposing abandoned building, or similar). and frankly, for you to buy yet another superzoom (if Canon were to release one), you might as well spend that same money on a consumer telephoto that will give you fast apertures at the same focal length, and better image quality.