Endgame spoilers below
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This is a speculation about what is going on in a couple of the "bad" endings.

When we give the tablet to Yeesha, it never appears in her hands. She paces around sadly and then links away. We find the tablet again, intangible, back in the keep.

My interpretation is that the tablet, like Excaliber, somehow tests the character of the person who touches it. If it finds them wanting, it resets itself into intangible form in the keep chamber.

Yeesha could once wield the tablet, but something went wrong and she could no longer touch it. She hoped she would have a second chance, but it didn't work. That's why she links away without a word. We can go back to the reset tablet, but now we have proved unworthy also and it remains untouchable to us.

Esher probably went through the whole quest, only to be rejected by the tablet at the end. He conceived a clever scheme by building a locked table in the library of Myst. If he could trick a person into completing the quest and placing the tablet there, he could write something on it to allow him to use its power. I noticed he never touched the tablet itself with his hands, only with a pen.

How did other people interpret the endings?

Well, I felt that the fact that you had to let go of Myst to get the good ending was symbolic. Letting go of the past, the franchise, and going to Releeshan, a bold beginning. With the stunning view of Releeshan, it didn't feel like an ending.

What we played in Myst wasn't the story. Gaternay, Terahnee, D'ni, Veovis, Gehn, Ti'ana, Atrus, Yeesha, Myst, Riven, Bahro... that was just the Prologue.