I don't see where you think it adds people in attendance when Opening Day was sold out. Do you have stock in Stub Hub? You do realize the people can sell their tickets on other venues?
I do agree Yankee face value prices are through the roof but you're angst against them not using Stub Hub is crazy. There are other options where a 400 dollar ticket can be sold for $10
Sold out doesn't mean those tickets were used to enter the gate. Plenty of games are sold out with 20% of the seats empty, more so now because ticket holders (mainly season ticket holders, but also brokers) are limited as to where they can sell their tickets on the secondary market, which makes it difficult to dump tickets on short notice.
So yes there are other option, but you can no longer use paper tickets, so you can't print at home. In addition, you can't use the digital tickets unless the tickets are through Ticketmaster, the Yankees ticket exchange system. All of this is intended to suppress the secondary market.
I had season tickets years ago and having the ability to sell them on short notice was very important, as was the ability to sell of random games I didn't want to go to in order to offset the total cost of the package. This is a bad deal for fans and the Yankees should be making decisions to entice and encourage fans to come to games, but this does the opposite.