Your post is a perfect example why formal contracts with players are signed to protect one's investment.
The club invests lots of $$$ into hopefully talented, yet completely unknown young players - like a 11-year-old Tomas Berdych in 1996 or a 16-year-old Petra Kvitova in 2006, while hoping they will develop a new tennis star and get their money back.
But they know that when their player breaks through or even wins a Wimbledon (they have obviously learned their lesson - see the Hingis example above) that other agents/managers/clubs will want to entice the new-born star away and they may lose a lot of money.
Thus, e.g. Tomas Berdych's contract includes a clause that he must not play for a different club (at least for a certain period). Petra's contract should also contain some restrictions or even sanctions if she suddenly leaves and relocates.
PS: If Petra wanted to she could get out of this contract easy (even in the Czech Republic). Last time, I told you practically speaking (Petra leaving the country, PR
at home, how the courts favor the talent, etc), and how developing artist and athletes is not easily quantifiable. OK. Let's look at it your way ("everything you claim Cernosek has done for Petra").
If Petra said "I don't want to be with him anymore", and they went to court. The courts would ask Cernosek to provide all the developmental tennis cost he spent on Petra at Prostojev. And let's even say he said $200,000, and Petra agreed on all of it. OK, so they would ask Petra to pay the $200,000 and maybe another $100-$200000 for his services and/or future earnings (within a reasonable time frame). Thats it. Petra could buy him out for $250 to $400,000 (probably less, but I'm using your Cernosek friendly example). And she's gone. Keep in mind, Petra can also say XX $ Cernosek made off of her, helped pay back those development cost in the first place. But I was using your straight Cernosenk example.
Now, how else would Cernosek have a future claim on Petra? What could he provide her, if she wanted leave that she couldn't get somewhere else? She doesn't need him to play tennis. And She's not a slave or forever indebted to him. I'm glad you said the word "investment". OK. Petra would pay back his "investment" (if that) and move on. Think about it? That's all it would take (generally speaking). That's how the court works in these cases.
This is not like an Actor and a movie company or broadcaster; a artist and a record company or a NBA player vs his team.
Those institutions actually PAY HIM or HER (the talent) a lot of money for the artist to leave would have to buy them out or forfeit payment on their contract $$ by walking away, and/or getting sued for it. Managers and Agents only RECEIVE smaller percentages/payments, not generate the actual income.
Plus, the Record company, Broadcaster, Movie House, Sports team/NBA provide him/her the distribution or apparatus (THE NBA, TV station, record distribution and radio play, and Movie house distribution and promotion, etc.) which prove their se value to the artist/athlete. This is what I mean when I say Cernosek is not a content provider or distributor, and is only ancillary to the process. So it wouldn't be hard to walk away from him, or for Petra to BUY OUT Cernosek in comparison. That's why movement happen so frequently with athletes, artist and their managers and agents. Cernosek provides a service like anything/anyone else. And if Petra doesn't like it, she can freely move on (unless there's money owed and that's worked out). She's not forever indebted to him. Anyone can say they manage someone successful or famous. How hard is that? They would have to prove they provide a service essential for future payment (such as cutting certain endorsement deals). However, he'd have little claim in Tennis, cause Cernosenk is neither the talent or the distribution, so even if he claimed he helped assist her development, it means nothing (or not much) to her success. And even with the endorsements, the courts would lean more toward the artist/player if they were already successful and famous, when negotiated.
So if Cernosek negotiated any endorsement deals for Petra--outside of Tennis, and could prove it/that he was essential, sure he would get XX $$/% of those deals and they could continue as they are till they expired or Petra could buy him out of those as well (depending on the amount of payment expected and if she wanted to). And the same goes for her appearances (if they're even written into the contract) to play at Extralagia, Prague 100K, Fed Cup or any other Cernosek sponsored event. Not hard to finish/fulfill or to exit from.
So, in my opinion, this wouldn't be as hard as you think Petronius. And the tennis management part would be SO easy. Honestly, Cernosek wouldn't have much of a leg to stand on, if Petra wanted to leave (even in the Cernosek Crony Czech Republic). Case closed. And of course, she could make it even simpler, and just leave the country altogether.