Although far to be a total victory, Qubic's merge mining attack on Monero shedded light on a weakness for pure PoW blockchains. Indeed, merge mining enables an auxiliary chain to incentivize the concentration of the hashpower in exchange of extra rewards. In the case of Qubic, the flaw is more apparent as Qubic has been intentionally adversarial but the same issue would remain for other non-adversarial auxiliary chains such as Tari or DarkFi. Because of the extra rewards offered, it would be rational for economically motivated miners to direct their hashpower to those auxiliary chains, hence concentrating hashpower over time.

Maybe some smart brains will figure out a solution that would prohibit merge mine on a pure PoW blockchain but assuming this can't be done, a PoW/PoS mechanism could be an alternative solution.

It's exactly because of security concerns that some PoW blockchains have moved to a hybrid PoW/PoS model.

eCash (a fork of BCH by Amaury Sechet, the founder of BCH), has moved to a hybrid PoW/PoS (Nakamoto+Avalanche consensus) to prevent 51% attacks on the network and improve the user experience (sub 3 seconds finality). The goal of eCash is to be the best form of digital cash which requires fast finality. Still in the case of eCash, it can be debated whether or not digital cash with optional privacy can be the best form of cash (to most Monero folks, the answer would be no).

Another example is Boolberry that was relaunched as Zano with the migration from pure PoW to a hybrid PoW/PoS chain. Here again, security concerns motivated the transition. On the user experience front, Zano also benefited from the integration of PoS by offering faster transaction finality. Notably, it's likely why Aaron Day chose Zano over Monero for the launch of his point-of-sales system as long finality times aren't acceptable for in-person merchant payments. It's questionable whether Zano is secured enough with PoS as the coin distribution was heavily influenced by the Boolberry premine, but this is not an issue that Monero would have.

Due to its fair launch, focus on medium-of-exchange and lack of supply held on exchanges (thanks to the delistings) Monero is really well positioned to augment its consensus with PoS without fearing attacks related to the concentration of XMR in the hands of a few. PoS presents the advantage to lower the barrier to entry to participate in the consensus and earn a share of the coin emission. It should make the network more resilient to the attack of a small actor (let's be honest, Qubic is a small actor). Plus some PoS consensus such as Avalanche can allow for a high degree of coin concentration without risking the network of being attacked. Even with a classic PoS consensus, Monero would certainly be one of the most secure PoS chain out there.

In addition, PoS would enable faster transaction finality which is a key feature Monero lacks to be the best digital cash possible.

That said, PoW still has its importance for Monero. In pure PoS blockchains, a new validator joining the network needs to connect to a set of trusted validators to load the blockchain history. Those are usually maintain by the core teams or foundation. The real utility of PoW is to enable a new validator to bootstrap the blockchain in a trustless manner (by seeking the chain with the most work rather than trusting a given set of validator). Hence a PoW/PoS model is preferable to a pure PoS model.

It's no secret that the culture of the Monero community is generally opposed to PoS. Maybe this strong stance is slightly ideologically driven. We certainly can be proud of being one of the few respected PoW blockchain left out there but maybe this Qubic event will change the narrative. Whichever path Monero takes next, hopefully the chain will gain in resiliency.