I'm sure lack of confidence has a big role to play in Rogue's slump, but the way I see it is Rogue became too predictable. This is why teams have an analyst and/or do VOD reviews. They start picking up on patterns teams use to engage in fights, among other things. Rogue dominated the NA scene for quite some time, and teams definitely looked at their games the most to know why and more importantly why.
I also don't think it's fair to put Vallutaja and SoOn on the same boat, for the sole reason that the latter is a constant in terms of pulling through when the team would look totally disjointed.
Soon would always be on the backline getting the first pick in matches and winning them fights.
I think this is a good point you brought up as I noticed teams being way more agressive against Rogue. Here's my take on it.
Obviously a player can't become bad (or not as good if you want) in a matter of weeks. Logic would want this to be more of a team failure to renew itself (or, and imo more accurately, enemy teams upping their game). You can't, especially as a flanker, be effective in the backline if the rest of your team is getting outpressured.
All of us here know that aKm and uNKOE will stick together, or that Knoxxx will primal rage to buy cart time if need be, etc. There are many more small details that we can't pick up on unless we do an in-depth analysis of Rogue's games.
All this to say that SoOn not being as pesky as before doesn't necessarily mean he lost his edge, only means they're getting outplayed because most of the teams can see them coming.
As for the solution, who knows honestly. Maybe bringing in some new faces will help them find that versatility they desperately need, maybe they'll grow stronger by sticking with their group and pulling through the slump.