In terms of Bayern failure to maintain their dominance this season, I'm more inclined to think that it's purely due to Leverkusen impressive performance, we can't blame Tuchel entirely. I think if Tuchel can bring Bayern to collect more than 71 points at the end of the season, it means that statistically Tuchel is still better than the previous coach. Last season Bayern won the trophy with 71 points, currently with 66 points and 4 matches remaining, Tuchel has the opportunity to take Bayern beyond last season points tally.
You're right. The failure of Bayern Munich this season wasn't purely caused by the bad quality of Tuchel, but it is caused by the outstanding performance of Leverkusen. It is difficult to expect winning Bundesliga trophy when Leverkusen never got a single defeat until now. Bayern Munich can collect 78 points if they win all the remaining matches. Sure, it is not so bad stats, it is better than the previous coaches. Tuchel actually deserves to get a second chance in the next season. However, it totally depends on Bayern Munich management.
The luck of a team like Leverkusen will be very difficult to control, not only in the Bundesliga, we also know that Leverkusen is also very consistent in the Europa League and DFB Pokal. Luck is on the side of Xabi fleet, where no team has been able to beat them.
It is not a luck, the luck won't last in the whole season. It is the result of high commitment in building Leverkusen to be a stronger and established team. Leverkusen can perform consistently in all competitions (Bundelisga, Europa league, DFB Pokal). If it is just about luck, Bayer Leverkusen won't be unbeaten in all the competitions.