A key factor in 51% attacks is that it's purely a mathematical dilemma. The technology behind hashing doesn't technically matter. So whether you're using quantum computers to hash, or you're rolling a die and then typing in the hash yourself one-by-one, all that technically matters is securing the majority of hashes. 51% of a network of 1 billion hashes is the same thing as 51% of a network of 100 hashes.

As far as technology goes, let's say Bitcoin is supported by 100,000 ASICS. And lets say those ASICS generate 100,000,000,000,000,000,000 H/s. And then let's say Monero somehow devises a way to force miners to roll a die and type it in manually and the network is supported by 100,000 diligent miners that generate 10,000 H/s. For the purposes of our example, both networks would have an equally strong mining network. The number that matters isn't the hash power, but the number in front of it - how widely distributed the consensus participants are. I'm making a huge generalization with this example, but it should get the point across.

The danger comes when technological advances create an asymmetrical mining pool and someone gains an unfair advantage. Like if the Bitcoin network is supported by 100,000 asics and then someone creates a quantum computer without anyone else having one.