Where your reasoning goes wrong is that the timebase knob is outside the field of focus of your eyes but the touchscreen isn't.
Well, with the practical experience of trying to do adjustments on a touchscreen control while looking at that same touchscreen for the results (on a mobile phone and couple large tablets), I can say that a physical control devices win 10-0. Either the touchscreen control is still far enough away from what is being looked (even few cm is enough), or it is too close and hand/fingers cover the thing being looked. This could of course be something that might be learned over time, but so far, few hours on touchscreen and I was still just as annoyed with the experience, vs. keyboard and mouse combo for the same was easy (but of course, been exercising mouse usage for 25years). Or using a knob on a scope was intuitive and easy after less than a minute of learning the sensitivity.
For that kind of activities, it is currently not even a contest.
Once the knob is between fingers, one typically does not have to look at the knob to make adjustments, but making changes on a touchscreen control will sooner or later end up with finger slipping off the control (and in worst case, on top of another control, messing things totally up). There are modern somewhat helpful software tweaks for that problem, though, but to expect the electronics/measurement industry to bother with them (yet) is quite the daydream, considering at how crappy level they still are (compared to e.g. the best experiences on mobile apps or PC software that have actually bothered to work properly on the usability), even on the expensive ones. I guess it comes from the development cost distribution; when hardware development is so expensive, there isn't that much left for software, whereas mobile/PC software devs can put everything on just that software.
However, for actions like menu handling or selecting parameters from a list, a touchscreen is much more convenient, even without any touch feedback (as long as the visual response is "instant").
So, I'm fully expecting touchscreen scopes to become more common over time (considering that the touch layer is quite cheap thing these days), and hopefully softwares gets developed so that one can choose (on that same scope) whether to touch the screen or buttons/knobs, whichever happens to be more natural for the user and action.