I agree with that, GOG's predominant unique selling point is of course DRM-free and without that the incentive to shop here goes way way way down. While everything on Steam is not DRM laden, I would think that most people who prefer to shop here not only prefer DRM-free but also prefer the ability to direct download from the website and some even feel that just having to use the Steam client to download and install a game is itself DRM. Without mincing words or debating about that, it can be agreed that some people find the necessity to use the Steam client to be an inconvenience and would like to avoid it, and so that is a GOG selling point as well.

For those people who either don't care about DRM, or whom prefer DRM-free but still buy some things that may have certain types of DRM, they generally have no problem shopping on Steam and if their choice is between buying a game with DRM on GOG or one on Steam, as it stands right now all things considered in the DRM-laden world Steam has a lot more to offer in terms of service and convenience to a lot more people than GOG does, so having DRM content on GOG would not only cause a massive boycott by a large number of users, but the remaining potential customers would likely buy DRM content from Steam instead of GOG anyway, leaving GOG with a much much smaller piece of the overall pie.

In short, DRM content on GOG would not likely sell too well for the products that have it, and it would have a negative impact on all other product offerings that don't have it due to the inevitable mass exodus that would occur. As their pie slice shrunk, there would be less incentive for game developers to consider releasing games here at all period let alone DRM-free, and GOG would likely shrivel up into nothingness not long after.

So I don't really worry about DRM coming here for that reason, it would be a death knell. If GOG's business ended up doing so poorly that it came down to that, they would more likely just go out of business than attempt to sell DRM content IMHO, but regardless of what they chose they'd probably go out of business either way by choice or by loss of customer loyalty and market share.