Meanwhile I have come to the conclusion that it can be easily proved that as soon as bitcoins would become truly valuable, it would be lucrative to fraud the system by gaining more than 50% of that cpu-power.
When they become truly valuable, transaction fees will be worth far more than they are now, which will increase difficulty, meaning the cost of attaining 50%+ of the hashing power will be far higher than it is now.
It would also never be lucrative to fraud the system through a 50%+ attack because it would reduce the value of the bitcoins you have.
In the long run however, blocks will only be rewarded with transaction fees and (a market equilibrium will form where) the cost of producing the hashing power needed to find a block will be equivalent to the total of transaction fees in that block.
The cost of producing hashs is not a short term cost. It requires a long term investment in the hardware that produces them, so unless there's a way to double spend for hundreds of blocks without crashing the value of bitcoins, it would not be worth it. It would be more lucrative to just be honest.