It's just logic for how animation works. Yes, there can be some studios / products that have better animation than others because some animators are better than others, but thing is, animation is easily affected by budget. You can throw twice the in-betweeners animators with twice the money and have twice the number of frames drawn in a given episode.
Well, in this particular situation you propose a
good scheduling is still the limiting factor! Because, consider the following; do you think every sheet of 'dōga' worked on by an in-between animator is going to be deemed usable? Well, not at all, and given your reasoning more staff with an in-between checker position is going to be needed. But, the in-between checker sometimes needs the animator to do the corrections in
time, right? The in-between checker signals them and returns the sheets to the in-betweener in question. All that without taking into account how first the key animation must be definitive in
time to serve as clear guidance, because at that point more key animation directors should supervise in
time the correction for the 'genga' itself... and what if the key animation needs to be returned too? What about having the necessary layouts first? We have witnessed by now how little successful many productions lately have been throwing animation directors at a problem of clearly deficient planning.
Every step of the process is a continuous back-and-forth, and incorporating more people into the production line can make it more unmanageable too... so, with all of the above in mind, we have that the time required must be clearly a common denominator of the process in a way that lack of such key resource is ultimately the decisive element. Of course, "time is money" as well, although money alone isn't gonna solve a whole lot, I suppose. Also, having "twice the number of frames drawn" can't be a particular benefit without a clear or apt direction or expressive intent behind the cuts in question. Limited animation is an stylistic approach too, and the timing isn't a concession but a methodology to master on its own, with several attributes perceived depending on the specific situation and results wanted.