Would you pay a subscription for access to the old games? Ad revenue is recurring and is probably more attractive to Sega (vs. a one time purchase fee).
This is no different from every other phone manufacturer but Apple. They have no reason to support older phones because it doesn't make them money. It does benefit Apple because they get recurring revenue from the App Store.
You might be willing to pay 20-40 bucks for a game, but the vast majority of users aren't.