They don't need to monetize it. It's a retention-focused game mode which feeds into MUT already.
As long as users are playing franchise games in Madden - which there are many, 20 a season or thereabouts - EA's servers can feed users playing franchise games Ultimate Team advertisements. Those MUT ads are splatted squarely in users faces whenever they boot up and in between-games loading screens. I'm sure you've seen them plenty of times if you've played any of the past three Madden games. In addition, the MUT tile is prominently featured in the front-end menu and users will see it any time they wish to return to their franchise. That's not an accident.
Between the already retentive nature of franchise (most users will not complete a full season in a single Madden session), the loyalty of the franchise fanbase (there's a lot of people who play franchise in Madden), and those ads and menu designs: if even 1% of franchise players see a MUT ad and get inspired to go deep enough into MUT to buy a pack with real money, that's worth it for Tiburon. That's a converted user.
That said, EA needs to give those loyal franchise players a reason to keep playing, and sabotaging those players' favorite mode will only make them drop out of the game outright. To that end, franchise mode ain't going anywhere, it isn't going to fall off the development radar, and it remains important to the monetization strategy of the game even without micro-transactions.