"Leave it to Congress" is tantamount to deregulating literally everything.
I understand there is an infinite number of possible policies, but in a representative democracy there are usually only a couple possible policies to choose from.
What is the actual thing that will happen if the Supreme Court rules one way or another in the Chevron thing?
Very roughly, the Chevron ruling ensures that federal courts give deference to executive agency policies when those policies are reasonably based on the law passed by Congress. Federal judges, lacking detailed knowledge of some specific set of regulation, cannot impose their judgement during a case - they must defer to the agency.
Overturning Chevron could allow lower-level, unelected judges impose their personal views when hearing cases (vs deferring to the agency tasked with enforcing laws). The argument that an unelected bureaucracy is "making law" doesn't pass the sniff test - those agencies are reviewed by Congress and limited by law. Federal judges have no such oversight - our primary recourse when they go off the rails is impeachment.
Super-simplified example... Boeing doesn't want the FAA to regulate airplane doors, so they sue on the grounds regulating doors isnt explicitly listed in law as a something the FAA can regulate.
The judge hearing the case believes Boeing should be able to build doors however it wants and rules against the FAA (under Chevron, judge would be required to defer to FAA's industry knowledge).
Impact: Boeing can now install doors with fewer bolts that required without risk of being grounded by the FAA.
And the Chevron doctrine is not "unelected agency heads rule your life" - remember those agency heads can make rules because Congress said they could. By doing nothing, Congress implies approval. If Congress doesn't like what they're doing, Congress can easily pass a law overriding it or even abolishing the agency.
Figure that out, and then you’ll figure out why Congress doesn’t just make all the decisions.