They have Giannis, they have Dame, they have good defenders in Lopez, Portis, Crowder, they still have a 3rd shooter in Middleton. How can this type of roster win only 49 games. Are they saying that they will only get to at least the 4th-8th spot in the East?
They've been the top team multiple times in a few years, and for me, they just got better offensively with the acquisition of Dame. Yes they lost an all-around player in Jrue, but having a sharpshooter in Dame can give headaches to other teams. He extends the defense of their opponent's teams because of his efficiency in three-point shooting, and because of this, Giannis will experience double teams lesser times than the usual, and we know how he bullies his defenders inside the paint right.
If there's one thing that makes this prediction realistic, that's injuries. If Lillard "AND" Giannis unfortunately gets injured, and no one can step up, that I will agree to ESPN's prediction, but aside from that, I agree with what you said that they might get close to 60 wins or even higher.
49 or more wins in the upcoming regular season will likely be an easily attainable goal for the Bucks. Of course, injuries are something that can ruin any team's plans. However, I think the probability of Lillard and Giannis getting injured at the same time is extremely low because both are not as injury-prone as Anthony Davis, for example.