Headcanon: In exchange of a power boost, increased IQ and new moves, an Alakazam’s body suffers greatly. An alakazam in the prime of its life will have the body of a decrepit old-timer. The additional power it obtains is usually used to keep it’s now-weaker body together and to sooth constant problems such as:
Joint and muscle pains, frail bones, a weak neck and spine, keeping a massively heavy head afloat, maintaining balance that the tail used to obtain, etc, etc.

Without psychic powers, a normal Alakazam will be helpless, and a mega Alakazam could die without constant psychic powers keeping its vital organs running.