I agree, just because Leverkusen shocked the world once, they all assume that it could happen again. But the reality is easy, we are not going to see that happen yet again. It's just not that possible and it's not going to be easy for any other team to ever achieve that.
For Bayern to lose the league, not only Bayern needs to play horrible, but we need some other team to get a great season. Let me put it this way, Leverkusen didn't lose a SINGLE GAME, literally zero, on the whole season. Look at the season they won, you will see, they have ZERO losses, think of how great you need to be in order to get something like that. So you need a team to be perfect like that AND Bayern to do badly, in order to make this a reality. We all know that is less than 10% chance of it happening.
Seasons like this happen once in 100 years, even Bayern hasn't had a season where they haven't suffered a single defeat. Plus Leverkusen won the German Cup without a defeat. In fact, they had one defeat in the entire season, to Atalanta in the Europa League final. But the probability that all the factors will come together again and we will see a repeat of this is extremely small (how small can be judged by Bayern, which always dominates but does not win the golden double every season and never has a season without defeats).