Reducing the reward per block means that miners get even less reward. See, a higher hashrate means that more miners joined the network and thus the block reward needs to be 'shared' by all of them (actually one finds it and the others don't but over time it averages out). In other words: a higher hashrate already means that you'll find less blocks, because there is more competition. Your profit ratio as a miner goes down.

If the block reward would be further reduced, not only would they find fewer blocks due to competition, but when they do they would also find less Bitcoin. This makes it doubly less rewarding to mine Bitcoin, giving an even lower incentive to mine Bitcoin.

It's a precarious balance, and yes many people feel like this system can be improved. However it is hardcoded into Bitcoin and thus would require a hardfork (like Bitcoin Cash) to change this aspect. And also it would be a deviation from the Satoshi whitepaper, which is sacrilege in itself according to many.

After 22 years no more Bitcoin will be found, correct. So miners no longer receive blockrewards. However, miners also earn an income through the Bitcoin transaction fees. These fees are expected/intended to be big enough by that time to incentivize miners to keep validating blocks. Right now the miner fees have been pretty high already, because of the fact people are paying a lot of fees to get their transactions through. With more usage, fees are expected to be high as well.

Nobody knows for sure how this will pan out and if this plan will work in the future! It's all still experimental so we will have to wait and see..