If you have a young player that is doing better than your 140k per week player and they play in the same position, you can sell your player on 140k wages and open up extra wages and hopefully bring in a lot of transfer money for your club, so then bring in another young backup, maybe, in case your young 40k wage player gets injured, and have some extra transfer money to either save, or spend on building depth elsewhere...
I have the same problem, however. My players always want a high salary hike at some point while at the club, but I have a wage structure and I won't let a player go over my wage structure max unless they are super special, like the next Messi. Otherwise, they I try to negotiate it down, or sell them on and bring in someone else who is good and fits the structure, all while maintaining a competitive side. I normally offer a new contract to my players with 1year-6months left, unless they are playing very well and I am to keep them at the club for a long term and don't see them leaving anytime soon.