No, you are not really missing anything (but see note below).  I agree, it is messed up.

However, the taxpayer is required to tell the Marketplace that the child is no longer a dependent, which hypothetically means the child will get their own 1095-A and they can't keep doing this every year.

The only "catch" is that it is debatable if the child can actually qualify for the credit or not.  The child's income is under 100% of the Federal Poverty level, which disqualifies the credit unless certain conditions are met.  One of the conditions is that the Marketplace had estimated the taxpayer's income to be over 100%.  Well, the Marketplace did estimate the PARENTS' income to be over 100%, but not the child.  But the way the splitting/allocation works, it seemingly does not matter.  At any rate, the IRS has been allowing it to be done this way for the last 6 years.