My grandpa lived in a building with unnecessarily expensive local coal heating. They never plugged into a heat pipe from a nearby powerplant, because the neighbor, who operated the local boiler always vetoed the decision. Why? Because he was being payed for operating it.
This is pretty similar situation. Incentives of the miners are not aligned with those of the users.
The incentives of the miners are aligned with the users: The miners mine and then they get paid.
If the incumbent miners stop mining in protest of their future profits declining, then new miners will just come in to take the profits they are refusing to collect today. There is no incentive for them to conspire and every incentive for them to not conspire.
Delegated proof-of-stake is worse than Proof-of-Stake or Proof-of-Work from a decentralization perspective as the "delegators" can bribe their way to stay in power.
Some resources (they link to DPoS alternatives and discuss them):
This is pretty similar situation. Incentives of the miners are not aligned with those of the users.