There's no way the NCAA is going to allow tournament games to move behind the ESPN+ paywall. They're streamed on ESPN3 already (the free part) and a few sometimes are streamed exclusively every year, but there's a contractual reason they are being broadcast and available. That deal includes online streaming obviously but the NCAA would gain nothing out of them being moved to ESPN+ (if they wanted to put these games behind a paywall, you can bet the NCAA would do it themselves if they could and cut out ESPN completely).
Besides, even if they generate scant ratings, what else does ESPNU and ESPNews have to televise at the end of March? Hoops is over, the other winter sports are all done. A couple of lax games aren't going to get bigger numbers, so it's not like something significant is being pre-empted that's going to necessarily draw that many more eyeballs for them opposite NCAA basketball.