What is so wrong with a steam installer compared to a non-steam installer?
There has been a lot of posts in this and other threads giving various reasons. As I can remember the main ones can be summed up as:
* Practical problems (like Steam client working badly/not at all in some geographical areas, no internet connection)
* Steam client taking up resources (May possible be a non-issue if not needed except for install/patching)
* Ideological reasons - Even if you will not put computer-wrecking DRM in it it will be a third-party control involved, Steam becoming monopoly, etc
* Concern about playing the game years from now when both Steam and Paradox (really hope not..) may be gone
* Not trusting Steam - Risk of losing access to games based on some mistake, privacy issues etc
* Plainly not wanting to buy games as a service
* Worries about being able to buy games in one country and play in another (some people move between countries and have had problems with this)
* Worries this will prevent modding (but should be pointed out that developer comments rather say the opposite)
* Jump through hoops arguments that getting a pirated copy will be easier (yes Steam games are pirated just like all the rest)
* Personal reasons (previous bad experiences with Steam & their customer support)
* General feeling of Paradox turning to the dark side and becoming big evil business throwing customer relations out the window for quick $$$ (ok, may not be entirely Steam-related, but going Steam-only would in the eyes of Steam-haters be a start down the path of evil)
I probably missed some, but tried my best to sum up the answer to your question. And I think that if you go Steam-only you need to recognize that for many of your customers those are real problems to various degrees ranging from a minor annoyance to a complete showstopper.