But how is it in the future_(10-20 years), if the rewards goes down to a lower percentage and ada has already a very high value. why should people keep there investments in ada? won't this lead to ever ongoing volatility?

In honesty idk. I know that similar protocols (other altcoins) have infinite supply, thus they can keep minting new coins. With cardano's finite supply the rewards will naturally have to end at some point. Thereafter the rewards would only be fees in the protocol... I'm pretty sure that Charles is very much against having an infinite supply. In the end I think possible future issues would be solved in CIP's.

The amount of transactions will increase so that's the future income.