but there's really no issue if a player decides to moot a few matches.

That's a very strange point of view. In my dictionary game is something you try to win. Just try to imagine Manchester United deliberately loosing to Crystal Palace 0:15. Players and coach would be hated by supporters. Forever. And if players would be caught making bets on this faked game, it would become a criminal case.
Deliberately loosing matches for financial gain used to happen quite often in the past, still happens today in many sports.

if they want to swim to the top with a single splinter and then have to dive back for quests?!

Diving back requires giving up the games. Why not - same answer as above. Also, leagues are structured so that if a team is too weak it's relegated to a lower one. If players have stamina to play only one in two matches, team will drop down at the end of the season and whole next season play with teams which are at a similar level.

Moving into a higher league makes you facing much stronger opponents.

Well, isn't that exactly what should happen as you move up ?!

Yes, exactly, but not the way it is now. Current format resembles cup competitions, but they are knockout type, so there are no pointless games between cooks, waiters and carpenters who play football as a hobby and full blown professionals over and over. As it is pointless, boring and frustrating.