I have the impression that the MNT article is significantly over-stating the potential of this technique, at least with current technology. The mouse work by Xiaoping Ren (see this 2014 paper) seems to have a very steep death-curve post-surgery:
Forty Kunming mice and forty C57 wild type underwent the
AHBR procedure. After transplantation, 18 mice survived
for 3 h after the ventilator was disconnected.
There are no details in the paper about how long the most robust mouse survived for post-surgery. I would guess that it is not an impressive number, otherwise the authors would include it. Alternatively, the 18 surviving mice may have been euthanased after three hours - the paper does not make this clear, but it is common to have a predetermined time-limit for euthanasia in experiments where animals are expected to suffer from an irreversible treatment.
During these 3 h, the mice
awakened and displayed normal cranial nerve function and
characteristic responsiveness (blinking, whiskers moving, etc.).
After the mice awoke, electroencephalogram (EEG) recordings
were made directly from the cortex of the transplanted
Cephalons (Figure 3). Because blood supply was maintained at
an adequate level during surgery by anastomosing the donor
and recipient carotid and jugular vessels, the intra- and post-
operative EEG and ECG show electrophysiological activity.
This sounds a lot like a 'no' to 'can this technique be used to substantially extend human lifespans?', on current technology. If a human trial is approved, and is as successful as the mouse trials, the recipient will likely be dead within hours. If they survive for several hours, the level of motor control that can be expected in that time is likely restricted to unconscious motor tics (cf whisker twitching and blinking).