It is very decentralized because people who vote are decentralized.
block producers are equivalent to mining pools. you often get less than 3 mining pools dominating. often 1 completely in charge. DPOS fights against pareto principle by FORCING the block producers to split to lets say 21 EQUAL in power ones. no other system does this. then it takes 2/3+1 witness to make any changes, not 51%, so it's quite resistant to changes.
the most important is block validation, if you make more block producers, it will slow the network down.
other jobs can be given to others but then you're forcing people to vote for way more individuals which becomes harder. Dan's research shows it becomes too many and voter participation drops. can maybe do the proxy system.
you are being indirectly paid if you vote for good witnesses because you have to vest your balance to do so and can't withdraw quickly. if they do good job your vested stake goes up in value on market. if you vote for bad ones or bad ones get control, the value of your vested stake should drop because market doesn't like what they do.
We think that being a block producer is something more than just running a server. Checkout out candidacy for more details: https://steemit.com/eos/@marcinsteem/eos-green-decentralised-block-producer-candidacy
Imagine steem witnesses selling their witness votes and allowing people to be witness for a day :D just like bid bots let you get on trending for a day :D
Now I understand EOS constitution bans vote buying now :)