No matter what, then my stand is Arturia should have kept the original MS-20s Initial Gain, and then added the new Sidechain feature as an option.
I find this topic to be relevant.
Regarding you imagined example:
Have you read Arturias Adrien's explanation on the EG2 behavior here:
https://forum.arturia.com/index.php?topic=109489.0#msg190336 ?
Korg MS-20 apparantly have a known quirky behavior thats not as described in the original manual.
This article is trying to explain as well:
https://www.christeck.de/2021/04/03/korg-ms-20-envelope-generator-2-behaviour/With the avaible info in mind, then this would happen.
The EG2 Sustain does'nt modulate Filter cutoff amount.
And as your title for this thread say, then the EG1 would output full level regardless of the EG2 Sustain setting.
So still you would get the original behavior by setting the EG2 Sustain to maximum level in Arturias version.
In addition to this, then Arturias Sidechain feature actually offer you the possiblity to lower the EG2 Sustain, to get the result you seem to be looking for.
I assume it's the same no matter EG2 is patched to Cutoff, as it's allready hardwired to that.
EDIT: Using Arturias added new EG2, then the EG2 sustain does modulate the cutoff. So in this case i guees your inagined example will be correct. However that's an option you did'nt have in the original. Never the less, then this is an argument for having the original Initial Gain option. EDIT END