It's as simple as this: neither Isildur, nor anyone else, was capable of voluntarily destroying the One Ring. The only possibility would be another Maiar, and there's no guarantee they could resist the power of the Ring's influence and "survival instinct", for want of a better term. Even Frodo was unable to choose to destroy the Ring: in the end, he also succumbed to its power. You cannot overstate the power of the One Ring to influence the minds of others, especially in proximity to where it was created.

While Sutekh was attached/connected to the TARDIS, he was also protected by it; most importantly, he didn't age abnormally in the Time Vortex. When he was dragged through the Time Vortex on the rope, though, he was no longer protected by the TARDIS, and aged billions of years until he was finally destroyed.