Well, Switch will be an important source of income, but I’m afraid that the “mostly” isn’t compatible with the very fragmented scenario of sources of revenue that it’s necessary to build to make the project more profitable. I’ll show you why.
Here are TWP sales by store, at December 2018:
[Source]
The first thing that you can see is that if you focus mostly on any single source of revenue, you are practically rejecting money. While some sources are bigger than others and some sources are small enough to be temporarily put aside, no single source is big enough to motivate a publisher to ignore the others.
The second thing that you can see is that, while Nintendo was the biggest source of revenue, the Steam slice of the pie was too big to “not focusing on it”.
The third (and I think most telling) information that can be extracted from the chart is that the two larges sources of revenue for TWP, Nintendo and Steam, correspond with the only two stores where RtMI will be initially released. This is a confirmation that both sources are important to make the project profitable.
Fourth, considering that the publisher will focus initially on only two stores, in my opinion that implies that they should market the game in those marketplaces more, not less! The reason is that they have to compensate the fact that the game will not be intentionally sold to people who use only other stores (e.g. GOG). But this point is only a personal speculation.
And that’s why I would find it a bit peculiar if they would ignore a common promotional event in one of the only two marketplaces they have decided to focus on.
But of course, there are also some signals that support the hypothesis that they will not focus on Steam as seriously as they will focus on Nintendo.
For example, the sad condition of the forums is, in my opinion, telling. And the fact that, on Steam, Terrible Toybox has not published even one news to tell Thimbleweed Park users that a new Monkey Island game is in the works is, to me, incomprehensible and not even explainable by “not focusing on Steam much”.