Very short version of the long argument.

[I_am_not_mafia]

If we know to some degree something about the highest possible goodness there is a connection to it.
If there is a connection to it, it exists.
We do know something about the highest possible goodness.

This is where your argument fails. We do not know anything about any highest possible goodness yet alone that one even exists. Try defining what a highest possible goodness even is, which is the first necessary step to determining if it exists because you need to know what you are looking for.