They're out there, but ... Their reading habits are increasingly geared for smaller chunks in an electronic format. In effect, you're still trying to sell 'em big chunks in a hardcopy format. That's the key mismatch.
You seek to both (a) expand your paying audience and (b) market the magazine generally. Here's what I suspect to be a potentially effective strategy ...
FT has a large corpus of interesting and informative articles geared for the layperson. Your older material is currently dead weight, insofar as Dennis doesn't re-market it in any form. Leverage this resource toward accomplishing objectives (a) and (b) above.
Convert selected older articles into freestanding chunks suitable for mobile / Kindle (etc.) reading and make them available online for a modest purchase or subscription cost (e.g., $1.00 apiece for downloadable files; a cheap subscription to a growing library of old articles).
This is the sort of marketing motif most likely to attract and engage the younger crowd. If they like what they see, these recycled tidbits will serve as the bait to draw in new subscribers to the mag. In the meantime, the revenue will at least help offset the cost of trying this approach.