First, you just made the argument that the swigning of big names is irrelevant to fan interest. Whether Snyder signs Haynesworth or trade for McNabb "they keep buying the tickets and merchandise".
Second, you do realize that NFL merchandise like players jerseys are revenue shared with all the 32 teams? He makes extra money from merchandise bought through the Redskins' Pro Shop (just like every store who sells NFL merchandise), but every Haynesworth or McNabb jersey sold through NFL.com or any other venue not owned by Snyder has the profits divided equally between the 32 teams. Yes, he can make extra money from merchandise he sell at his owned venues for selling merchandise for marquee free agents he signs, but it is an extremely small fraction of what it cost to pay the guy to come to the team.
Third, the Redskins have tens of thousand of season ticket holders. Since they have the largest stadium, far more than any other team. You can't pick and choose the years you want to purchase these season tickets. If you don't buy them one year, you lose the season tickets. That means guys like Haynesworth and McNabb have zero impact on those tickets getting sold.
Fourth, fans might get excited about a big name free agent getting signed, but it rarely changes their habits towards the team. The most it does in most cases is for them to buy that player's jersey which as I pointed out doesn't really affect the bottom line.
Fifth, Snyder makes most of his revenue from things that are not affected by big name signings. His biggest piece of the revenue is from the TV contract which is negotiated on long term basis and divided equally among the 32 teams. He also makes a significant portion of his revenue of individual marketing and sponsorship deals (naming rights, etc.). Most of these deals (and all of the major deals) are also long term contracts and not affected by individual signings.
The fact of the matter is that Snyder makes a miniscule amount of money based on individual signings unless these signings propel the team into a consistent contender for a number of years. If adding Haynesworth and McNabb won him a Super Bowl or at least made them a consistent division winner, their additions would mean something to his bottomline. But both look to be gone by next season after two unproductive years from Haynesworth and one from McNabb.