If you think players will take more money to play with a struggling club, then you have not been paying attention. Things like what position they play, who the coach is, how successful the club is and whether they will be competitive for a premiership are also factors. If they weren't, then players would always take the big money - but fact is they don't. I mean seriously would a player rather sign a $1 million a season contract to play with the Titans or a $600,000 a season deal to play with Souths, Canberra, Roosters or the Storm? Most would take option b.
As to back ended contracts, the NRL have just about outlawed them due to the salary cap muck it landed many clubs in. Hence that is not gonna happen.
Again you ignore realities when it comes to player retention by clubs. They don't wait till the end of the season to finalise their rosters, and the players are happy with that system also. Hence your proposal that players would wait till after a promotion/relegation match to finalise contracts ignores current realities that neither the clubs nor the players will allow to change.
You can disagree all you want, but I am pointing out current realities. I'm not making this up.