Ahh, good question. I misunderstood you, apologies.
There's no mention of pricing, quotas and so on. Perhaps Codex will still be preferable for coding tasks as it is tailored for it? Maybe it is faster to respond?
Just speculation on my part. If it becomes redundant to 5.4, I presume it will be sunset. Or maybe they eventually release a Codex 5.4?
5.3 Codex is $1.75/$14, and 5.4 is $2.50/$15.
There you go. It makes perfect sense to keep it around then.