Changing lenses is actually no big deal and can be done rather quickly. I think you’ll be better off with two lens, both of which will deliver a higher image quality photo as well as a wider aperture, than one big lens with lower image quality and slower speed. A significant advantage of a dSLR over a fixed lens camera, is the ability to easily and quickly change lenses. By not utilizing that option, you’re not only downgrading the camera’s capabilities but you are also paying a lot of money for an important feature which you will not be using.
That’s a good point I hadn’t considered - i realize however that I’m talking to a group of photographers significantly more seasoned than the average so I’m wondering when you’re talking about lower image quality are we talking about say a 95/100 vs a 100/100 or a 75 vs a 100 (if that makes sense)
As a general rule, zooms with less zoom (say ... 3:1) will deliver a better image quality (IQ) than a lens with greater zoom (say 10:1). In general, prime lenses are sharper than zoo lenses. IQ encompasses many elements, essentially all the visual elements contribute to image quality. In particular, most low ratio zooms will deliver an image which is sharper, less distortion and possibly better color. Remember that sensor size, pixel size, pixel number and sensor type will also affect image quality.
On Safari, many of the animals will be active during the low light times of dusk and dawn. Large/larger aperture lenses allow much more light to hit the sensor. The aperture is like an iris, the bigger the iris the more light can pass through. In dusk/dawn dim light, a large apertured lens will make photography possible/easier/higher-quality. Less expensive cameras have less sensitive auto-focus systems which will have trouble or won't focus at all in low situations especially when combined with lenses with smaller apertures.
Large aperture lenses are called 'Fast Lenses' smaller aperture lenses are called slow lenses. Fast lenses are considered to be more of a 'professional quality' lens. Typically, fast lenses are larger, heavier, better built and significantly more expensive than slow lenses.
Unfortunately, your suggested rating system is simple and subjective ... unfortunately photography and photo hardware is very complex, multi-facetted and often boils down to compromise and personal taste. The are photo web sites which rate various lenses in the laboratory (objective) and in the field (subjective). But you have to know what the numbers mean. 100mm-400mm Lens A may be rated higher than 100mm-400mm Lens B, but the difference between Len A and Lens B may not be significant to what you shoot, how you shoot and how the final image is viewed ... and ultimately not worth the additional cost of ... say ... 50% more. Conversely, a different photographer may find the differences significant for shooting in a different genre, shooting for a specific look, shooting for a large viewing size and find the extra expenditure worthwhile.
There is no single, correct answer for your question. There is no perfect camera/lens that does it all and can deliver an image satisfactory for all. But, (the big but), most modern digital, interchangeable lens cameras are capable of taking care of your needs. I think the lenses you choose will be more significant in capturing the images you desire than your selection of modern digital interchangeable camera from a major camera maker, (Nikon, Canon, Sony, Olympus, Panasonic, Pentax, et al).
In general, the more you want out of a single lens (telephoto, portrait and wide angle) the greater the compromise. The less the compromise, the more equipment you will need. A tourist at a theme park may have a dSLR with one lens. While a photojournalist on assignment will have at least two cameras with a bag full of lenses.
(I hope I didn't bring more confusion to this discussion.)