As far as I know the Monero blockchain would not be a black box anymore from the moment on someone publishes at least parts of the data and calculations needed to break its cryptography. This would actually be quite bad from a privacy perspective, as Shor's algorithm would practically enable those using it to de-anonymize all Monero operations, at least according to this article. However, once this data is published (probably this will happen gradually), more and more blocks will become "auditable" in a more standard way, becoming "as open as Bitcoin".

Of course if only one entity (or a few bad actors) have access to a QC capable on running Shor's algorithm on Monero data you may be correct and this could lead to confusion, but I believe this timeframe won't last for long.

In conclusion, if quantum computing really becomes a problem, Monero will be probably even more pressured to change to post-quantum cryptography as Bitcoin. However, the "if" still is a big one. We don't know when it will happen and it can be still decades away or not happen at all. So I don't know if this really can be an anti-Monero argument. If it happens, both will have to change.

I'm still quite neutral rearding the BTC vs. XMR question; regarding the tail emission vs halvings question I may see a slight edge for XMR but not relevant enough to endanger BTC's future, as there are many possibilities to provide miner income as discussed in the earlier posts in this thread (I still mainly would advocate for sidechain-driven tail emissions ).