Generic supplements fall into two categories:
(1) Material that requires no mechanic support. Cumberland's
Fief is a good example of this kind of product.
(2) Material that requires mechanical support (stat blocks, etc.), but simply lacks it under the theory that the GM can supply their own stats for their system of choice.
The first problem is that you've eliminated from your audience anyone who buys supplements primarily to get plug-and-play stats and mechanical expansions (which appears, from my POV, to be the largest segment of the market).
Of course, the theory is that you're appealing to a larger audience across all applicable games.
The problem with this theory is that, while you are appealing to a larger audience, you are ALSO competing against a larger number of products.
Why?
Well, look at how generic products are used. If you're using System X, then you take the generic product and stat it up for System X.
Now, compare that to how a user of System X might use a product from System Y. What would they do? They would stat it up for System X.
And I postulate that people who are willing to use generic supplements are just as likely to be willing to use non-generic supplements from other game systems.
In this post-OGL era, therefore, there's no reason to produce a generic supplement of type #2.
What about type #1? Probably more feasible. Although, in practice, there's a very narrow niche of products that actually land in that area. And many of those are actually competing with non-fiction books and resources in general.
This is the calculus we performed here at DMP with our
City Supplements when 4th Edition was announced. We considered moving to generic, stat-less content in order to appeal to users of both editions. But then we realized that 95%+ of the books were already stat-less and the only thing we would accomplish by doing that was to make the books less appealing to players of 3rd Edition.
Personally, I land firmly in the "I'm willing to kit-bash from anything" camp and own several generic products.