It's clearly impressive and interesting enough, given how well they've done and still (basically) sell out most of the shows in most of their major markets.
Of course they
could invest more on production values, have bit of a different marketing strategy and whatnot that could/would result in growth, be it significant or not, but as it is, it's not like the Maiden machine is exactly struggling and honestly, I don't blame them (even Rod) for not optimizing their market value & viability for new/younger audiences at this point of their career. Besides, there is still a lot of young people attending shows in their biggest markets (e.g South America, Europe) and in that sense, they
do attract new fans in general, albeit obviously not as much as in their heyday, and their cultural significance and status is remarkable and I agree that they're mostly (=big masses of their audience) a "legacy band" for people who grew up with their music in the 80-90's, but getting new generations on board has been one of the most prominent factors of their "second golden era" around 2008.
Besides, I personally like how they've operated outside of the Metallica & co. scale of mainstream and although undeniably smaller act, I think Iron Maiden is very good at doing their own thing and totally a world-class act in any case.
As for the scale of their show, it's true that a lot of acts have surpassed it in a way, but frankly, Maiden's "boxed" yet theatrical and pleasantly subtle (with the lights, effects and all) show approach is (in my opinion) of course a lot more impressive than e.g Metallica's stadium shows from the recent years.
I've very much enjoyed seeing these "bigger" shows by Metallica a couple of times, but honestly, just the scale of the show doesn't really make it that impressive or memorable; the stage show/production itself wasn't anything to write home about, outside it's sheer physical size. Where as dry ice effect, lights, Bruce with a trench coat, giant candles & Scribe Eddie framed by icy Seventh Son style icy drapes within a "Maiden box stage" in 2013 = win.
I'm biased, I know.

I just don't see how it's even relevant that Maiden isn't growing much at this point, as their legacy & appeal seems to be mostly intact anyway and I don't see it dying out anytime soon, whether they'll continue touring for a couple of more runs or not.
Did, or will they ever maximize their market value & reach? No. Does it matter? No.
Will they still be remembered as a global phenomenon and a one of the most influential bands of all time with a legendary mascot & stunning live shows, along with the more or less truthful legend of being probably the "biggest underground band in the world"? Absolutely yes!