If every miner was also a node and vice versa, the mining centralization would get worse not better. You're creating a major incentive for miner centralization.

If every miner was also a node, then that means as soon as one entity controlled the majority of hashrate/block creation, then they also de facto control validation.

Now apply that to the current miner centralization and you'll see why it's a problem. You've eliminated one of the barriers to centralized network control.