Why? What difference does it make?
1. Steam updates for a time if you're not a regular gamer with Steam open all the time in the background, these updates take time.
2. You don't actually own the game, so in case some issue with the game arises, servers, country locks or anything else, you lose the game.
3. A mostly Single Player game doesn't need to DRM or third party overlay and launchers, it's not like we have cheating issues like in the competitive e sports scene.
4. If you buy the Steam version, and if they release a DRM free version in the future, then you won't be able to shift to it because all your investments, DLCs etc are in the Steam version, no one would want to buy the entire thing again., UNLESS, of course we could use our Steam Keys to access the DRM free version with all the DLCs.
5. Not everyone has access to internet 24/7, and Steam Offline only works for a day or 2, after that you need to log in online, update your Steam, and only then you will be allowed to play games. This for a massively single player game is ridiculous.
So based on all the above things, I'll suggest a things;
Most preferable would be a GOG or some DRM Free release,
Second, in case they're considering a later GOG or DRM release, it would be great if they could let us use our Steam key to access the DRM free version, otherwise those who bought Steam and wanted to shift to other version will be stuck.
Lastly, they never want a DRM Free or GOG release, please confirm it right now, so that we may or may not buy Steam, knowing that it was the only option available. I play only those games on Steam that have no GOG versions, so in the case of EU 5, I would appreciate this infomration beforehand buying.