Steam statistics are an important indicator on how a game is preforming and should be taken seriously.
Its all relative. If a game is exclusive Steam, then the Steam stats are an excellent indicator. If a game is multi-platform, but PC exclusive then the importance of the charts is somewhat diminished - well diminished by themselves. They can show trends, which can then be measured to sing telemetary, or platform apis on the other platforms. Now if a game is not even Steam exclusive on PC, the trends may show you a trend in the wder PC community, the trends may be only withi the Steam community.
Last set of data I saw, Steam was somewhere in the 25-30% of total sales (800,000 from 2.5K) - but that was some time ago. Steam growth is low, with most growth from other platforms in the last 2 years, so as a percentage Steam has reduced. Stll a healthy enough percentage to be statistically meaningful, the problem is sample bias. Taking from all one community, where there are many is not statistically as accurate as a cross section across all the platforms.
So my take is the Steam stats can show some trends, but evidence of a general trend requires stats from other platforms.
ED may well be dying, or it might be dying on steam. The latter might, but not necessarily mean the former.
As I am not Steam's biggest fan, the latter does not bother me, the former does.
Cheers
Simon