so basically they are going to kill the project in long term as the price is going to get dumped a lot harder after a while when the huge whales that own millions of ETH start dumping their accumulated free coins that they get just because there is a huge 10% profit for them out of thin air for doing nothing. and since they have virtually no cost (since it is no longer PoW and there is no mining involved) they can consider that free money as pure 100% profit. and that dump will crash the market on top of the ongoing dump that happens due to unlimited nature of ethereum and the fact that it already has a huge supply in circulation.
If I read the plans correctly, I wouldn't expect any dumps from staking for at least 1-2 years. As whatever is staked while on the beacon chain can't be withdrawn (I think).
However, long term may be a different story. I'm not sure how many will be getting that 10% or how many will be closer to 4%, I guess that could be a factor. If the reward system is based on number of coins held in staking, which is what I assume, the model may be closer to some sort of tiered masternode, so large holders may not be so eager to dump all staked coins, as if they did then their reward percentage would go down.