A checkpoint is in no way training wheels. It's for additional security.

Right. Perhaps comparing the coo to checkpoints isn't an apt analogy. However, the intent is the same. Without a coo, transactions would be probabilistic and confirmed once they went above a cumulative weight threshold, but the transaction volume is still too low to prevent a Sybil attack, hence the coo. It's still an open question whether or not the tangle can function coo-less, but it makes for an interesting experiment.