Consolidation does not appear to be something that we encounter frequently on these forums. It also did not appear relevant enough for LE to consider it when designing ARI (in particular, they need the replaced field for emergency revocations & rate limits). So if it's not deemed important enough for these use cases, I wouldn't deem it important enough for the renewal mailer either.
If it's deemed common enough, the entire ARI spec should be updated. I can't see an argument that says "it's fine for mission-critical use-cases, but it's not fine for the expiration mailer".
PS: I also don't see the issue here. Yes, there may be more or less rare edge cases where the "replaces" field isn't sufficient and there will be a certificate that is in fact replaced, but couldn't be marked as such. But where's the problem with that? In such a case, we might still have cases where the renewal mailer sends mails when it doesn't need to. But that's the status quo. Not implementing this means that the renewal mailer has a high "false positive" rate. Implementing this means that the "false positive" rate has the potential to drop dramatically, but it won't drop to zero. But who says that we need the mailer to be absolutely perfect with no false positives? It's not, currently, so why does it need to be now?