I feel like the more time you wait before playing a multiplayer game, the more the boss score get inflated. To a point where the up league has bots way weaker than the top of the under-league.
I would say that a boss isn’t supposed to get harder to pass.
Wouldn’t it be more fair to keep static scores on bosses? (immutable)
You can find bosses with 2+ inflation points. It’s unfair. 
What do you think?
Sometimes the boss goes up, and sometimes it goes down. It depends of the top of the league.
The best players, when they defeat lots of people but not the boss, make it going up. The only way is to be better than those players.
Agree, it’s harder to climb. But a static score boss means a certain trick makes everyone pass to the up league. What is the interest to be legend if half of the board is in it ? 
If the boss had a static score it would get easier and easier to get to higher leagues as people joined - any rating system with an increasing number of particpants suffers from rating inflation as the number of participants grows (technically the range expands at both ends of the scale but the lower end isn’t important here as we don’t do demotions). The other issue is that a static score would be very difficult to set to any meaningful value - a score set based on the state of the league before any promotions would be meaningless as the top X% of the league is removed. Currently the boss always starts with a low rating (as it has losses against those above it) which gets slowly raised up (as there are no longer as many losses as the better bots are removed) - this does make it slightly easier to get promoted if you are near the boss at the time of creation than afterwards.
There are issues with the way the CG system works (bots that are similar to the boss in skill level eventually get promoted as the ratings go up and down with other submits leaving a gap between the 2nd and boss), but a static score would be much worse.
I think a lot of CG multis are to various extents some rock/paper/scissors games. So if boss is scissors, a player at the top of the leaderboard is paper and a player that is submitting is rock, the paper player is likely to be pushed to the next league. Players that are pushed are generally weaker than players that get to the upper league by themselves which can look like “a lot of players on the above league are worse than the players in the lower league”. But well who cares, they had lucky matchups and got promoted, good for them.
Similarly, if a player that is good around the field but bad against the boss submits multiple times, the boss will be buffed. But in the end if your bot is in average better than the rest of the field and better than the boss it will pass as well. The main downside is that if you don’t totally wreck everybody, you might need to submit a few times before the boss elo is lowered enough. If you don’t consistently beat the rest of the field and the boss, you could either wait for your rock player to push you up, or just spend more time to improve your bot 
In my personal opinion, without the rock player pushing involved, I feel like it’s actually easier to get passed the boss when the rest of the field is weaker than when it’s at its peak (I think that most of my final bots that got to legend would not have made the cut on the Friday of the contest)