in the end, releasing mod tools nonetheless is a good move. Single player games generate their replay value and longevity through the quality of the game itself, adding modtools cannot virtually only lengthen the experience. Looking at titan quest for example, despite the mod community being relatively (very) small, it does happen that masterpiece mods like Lilith see the daylight. There might only be about 30 actual minimods for titan quest, but when i see a monstermod like lilith (up to 1.2gig in size) i'm glad it got modding tools.
I would throw in my vote to release mod tools in either situation