In canon, it's not even clear if TB's plan was sound. The Cathedral kills very high level characters on the regular. Presumably it draws its power from the Starstone itself, an artifact so ridiculously powerful that it doesn't actually make sense if you think about it too much. So simply nuking it might not do anything.
OTOH Tar-Baphon seemed pretty sure his plan would work, so what do I know.
Tar-Baphon's Wisdom score is 14 after old age and being a lich, so... high int, low wis?
(What I believe to be the breakdown for Tar-Baphon's Wisdom Score: 14 = 8 or 9 (base) + 2 (lich) + 3 (venerable age) + 0 or 1 (leveling up))
Also, no one tried it before, so he could have reasoned that it was a method that was likely to work, since
Crossing the pit is a necessary first step, but not sufficient to enter the cathedral and continue the test, and what has worked for one hopeful may fail for another.
And even then, honestly, who really knows what goes on in the mind of Tar-Baphon?