By the time their prospects hit their stride their current core of good forwards will be leaving their prime. On top of that the Pens and Caps aren't going anywhere, and the BJ's are younger and more promising.
I think this is an overblown idea. Sure, they'll be leaving their prime, but they're not going to stop drafting forwards and potentially trading for/signing younger and better ones. Good defense is a pretty finite resource, good forwards can be had. If (if...IF) the defense turns out as good as it can be, then they will be a very good team regardless.
It's not like the Sharks' top forwards are spring chickens, eh? Team is still a threat, sure they have Couture and a few other young engines, but they still give heavy minutes to guys like Thornton and Marleau. Thornton and Marleau are past their prime, Pavelski is 32 as well, but that doesn't render them ineffective players, it just means they weren't as good as they once were. Yes, Giroux and Voracek are expensive, but the cap will continue to rise and that'll become more manageable, and they'll have guys on bridge deals and ELCs also eating big minutes and scoring points. Do they not have Lindblom already, who should be quite good, and a top 2 pick that should be an 18 year old future top 6 center?
It's all potential, of course, but if you're granting that potential to a bunch of defensemen who aren't in the league yet, why also act like they'll never get another great young forward, particularly when it looks like they already have two in the bag in addition to a young and talented Konecny and a Couturier who isn't even 25 yet? And a pretty gigantic budget?
Basically, that narrative does not worry me one bit. Not like HF posters found the fatal flaw, I am quite sure the team is aware and is planning ahead.