I think that is true, not only to hold the motor, but also because the lens housing has to cover and support the full range of travel. In AI lenses the barrel would extend and contract depending on focus. Newer lenses seek to avoid moving around like that.
Which brings up back to the topic
Nikon's better AI-S, plus more modern Leica/Cosina MF glass, prove excellent results can be had while keeping glass relatively small. Zeiss has the quality, but their behemoth lenses could use weight reduction. As mentioned earlier, Chinese companies are starting to make some pretty killer MF optics, but their refinement still leaves a lot to be desired, and yet they're growing in popularity nonetheless.
Nikon has not made any advancements to its MF line for, what, 30 years?
With the craziness of the photography market, and the uncertainty of lifespan with any given line,
a MF lens line makes the most sense, even more so than an AF line, as MF lenses are the most likely to be capable of crossing over to a new make/model ... and are, functionally, more likely to "still be working" on an existing line 10, 20, 30 years later--and beyond.