Disabling the feature does not remove these features from the code, thus build time and space on the server are still increased for no reason.
Correct. Disabling a feature does not remove the feature's source code; it only makes the feature unavailable to the end user. Having a feature that is disabled does not negatively impact the performance of the application (because the code is never executed) and the blog and forum features combined only take up ~85KB of space on the server which is a very small amount of space. Regarding the build time, the Visual Studio compiler is highly optimized so the nopCommerce solution already builds fast. In addition, Visual Studio does not compile files that have not changed since the most recent build. I do not see either space or build time as a premise for moving features to a plugin. The blog and forum features add value by driving unpaid traffic through organic search results to stores that leverage those features which merits its place in the codebase. Stores that do not have a built in blog or forum turn to third party blog, news, and forum sites for the afore stated reasons.
P.S. Retailers use popular forums to already answer product related questions and/or to drive additional traffic to their stores. But, what happens if the store's built in forum becomes the #1 forum or #1 ranked forum in the search results (at least first page)? It means the store owner no longer has to post in the third party forums (although still could) but can post in their own to get more traffic.