My experience is that nicads will last nearly forever as long as they are in
use. Typically I'll recharge once or twice a week with universal remote
control batteries getting least at about every three months.

My experience, 25 plus years tells me they will NOT even come close to
lasting 25 years, even with normal use.

Leave the batteries unused in a draw somewhere for 9 months and they'll be
ruined. Recharging twice a year might not be bad advice.

Please site a link as to how a Ni-Cad becomes unusable (non
recoverable) after non use of 9 months.