But let's compare their VERY ENDS:Valkorion: his ultimate plan was thwarted in a snap by the Outlander realizing it was his mind, his rules. We're talking about an Emperor who's been essence transferring and eating souls for 1400 years.
Sidious: his ultimate plans were thwarted by the children of a man with Father-tier potential and the spirits of every single Jedi in history pulled him into the ultimate void of the force.
Was Palpatine ultimately thwarted by the children of the Chosen One though?
Valkorion was defeated by the greatest champion of the era (who took down all the big names of this era, including Revan and Malgus) and his own family. The circumstances of the defeat, as you point out, are absolutely pathetic, but at least we directly see multiple powerful forces unite to expel him from the galaxy.
Palpatine? I recall Palpatine running over to Leia, snatching a baby out of her hands, and then trying to transfer his soul into him. And then that floating ball, not a Skywalker, intercepts his spirit and brings him into the void. If it ended with Dark Empire 1 with Leia and Luke uniting to kill Palpatine, I'd completely agree with you. But we got that floating ball Jedi, who is so irrelevant in Star Wars history that I don't even know his name, being the ultimate reason for his death as far as I can tell.
And about that all the Jedi thing. I've always seen that as a minor quote in the comic that the audience has magnified in an attempt to overshadow the humiliation Palpatine just suffered. Palpatine getting swallowed by a ball doesn't look as bad when all the Jedi hold him down in the depths of hell, obviously. It's canon nonetheless, but just something I wanted to point out.
My issue is that even PEAK PHYSICAL VALKORION was getting thwarted and defeated for what seemed like forever. Meetra could have literally killed him with a saber throw in Revan. Hero of Tython and his droid fight through Vitiate's Uber nexus, give him more time to recover while expending energy fighting MORE elite guards to save his apprentice, and still resists Vitiate's TP and whoops his ass. Chapter 1 was all we saw of the great physical incarnation of Valkorion, before he was...back stabbed? ermHell, There's a codex noting that Valkorion/Tenebrae has been cut down/killed multiple times.
Yeah, this is an injustice Palpatine generally didn't suffer from. With the exception of his final death, all of Palpatine's deaths or near-death events were battles of extreme important.
Valkorion was generally portrayed as invincible, but not invulnerable.
Palpatine? In many ways, he's both.
I think that's a key distinction you're getting at here, which I have to admit. mmm