The Nats use dynamic pricing for their tickets. And they use Stubhub. It’s not clear to me why extra Season ticket holders would drive down prices. Those tickets are being sold on the secondary market either by the Nats or by a plan holder. Plus a person actually having a ticket makes them more likely to go.
Also, the Attendance number is based on ticket sales, not turnstile clicks.
I think folks are suggesting that the extra season tickets sold will lead to the secondary market being flooded by folks who really were not intending to go to too many regular season games. I suppose if it is a decision between "should I try to resell the ticket I have tonight through my single half season seat plan or should I think about going to see the Mariners mid-week," I'll guess the resale value will be limited, so maybe that person goes if it is a nice night, or maybe they just say "FP kind of grows on you when you are on your 3d gin and tonic."
Bluestreak - re: your immediately prior explanation of your thoughts - I don't think the Nats will ever sell a ticket below face unless it is in a group deal. You'll get 2:1 offers, maybe liberalization of RCRs for specific games, but never below face. Flooding the secondary market will lead to sales below face there. It's like what I used to do with an extra pair - sell a $130 pair out of a quad to reseller outside the stadium for $20 so I get my beer and dog covered. I'm out the $130, so any money is better than none.