Is bitcoin sustainable? A rather dumb title, however I have question that I cant get a grip on related to bitcoin:


If the amount of bitcoin is supposed to have an upper limit, what is the incentive to mine in the future if that limit is reached? 
If one entity controls more than 50% of the network then the protocol will break down, doesn't the continuous rise in difficulty destine that only one ASIC miner will be left? 
Who is it that sets that difficulty level and why then is it  called decentral? 
To participate you need the complete blockchain stored locally. How is this supposed to scale when data to store is growing exponentially?


I seems that the system is built to only day just vanish.
I'll address your points individually:


Bitcoin mining in the future will be supported by transaction fees rather than the block reward, see this question for more details: What will happen to mining after the 20 999 999th Bitcoin?
Not sure what you mean here, why would an increasing difficulty lead to only one miner? If people stopped mining, the difficulty would go back down again because the hash rate would have decreased, so there's an equilibrium point based on how profitable mining is at any one time
The difficulty is just an algorithm which is based on the time taken to mine the last 2016 blocks (technically only 2015, because of a bug, but that doesn't matter). So everyone agrees on the difficulty, everyone can calculate the current difficulty by looking at the blockchain. No one person sets the difficulty, everyone does.
Scaling is an issue which is a big topic at the moment, because yeah the blockchain is getting very large. But please note that it's possible to run a pruned node which only stores some of the most recent blocks and doesn't all the way back to the genesis block. Full nodes never use this historical data anyway other than when serving brand new nodes with blocks to get them up and running, and when rebuilding their UTXO set, which normally is only done once when the node first starts and doesn't need to be done again.