The two hobbies are too disparate. You may as well combine Stack Overflow and Server Fault since they're both "about computers."
This WotC survey shows that only 43% of RPG gamers play a classic type board game monthly. It's less than crossover with computer games. If there was cross-interest, more people from board games would be here already and vice versa. If there's not, an attempt to merge simply in order to "add the stats up" is ill-fated.
Basically, I have no interest in board games, I play RPGs. If this site is full of board game content, its value to me is extremely diluted and I would go back to all the RPG forums that, oddly, have plenty of membership without combining all those other games into them. The number of people that play board games/card games is way, way larger than those who play RPGs (3% of the surveyed population) and our content would be drowned out in the long run.
I think the response to low growth numbers isn't to dilute our content further - one of the goals of a SE is to curate good content to attract experts on a given topic. There are many RPG forums out there with hundreds of posts a day and thousands of users. The solution is to attract them, not become so defocused no one in their right mind would come here for RPG discussion.
Furthermore, I believe large numbers of board gamers couldn't help but approach RPG questions from that board gamer mindset, which means even the quality of answers on RPG questions would be biased.
RPGs are in general collaborative not competitive, they have a facilitator not just players - except inasmuch as some of the newer games obsess with selling tabletop widgets to up their revenue stream, there are few meaningful comparisons between the two.