None of these have any impact on privacy if users of Bitcoin are not using these features.
Obviously
When he wrote his paper, transaction fees were >$20, and using multiple change addresses would be very expensive for a business that processes many transactions. The same is true if a business generates transaction inputs in not the most efficient way.
You're right, even the paper mention high fees when he wrote his thesis (see 4.4.2)
But what i implicate/empathize is default privacy on Bitcoin (without CoinJoin, mixers or any method which require another people) is more complex/private that what's written on his thesis.
Above all, the most effective way to maximize privacy when using Bitcoin is to abstain from address reuse, and to only conduct business with those who abstain from address reuse. This would be very effective in making "mixers" obsolete, and unnecessary in most cases.
Prevent address usage/one-time address is what everyone should do, but it's not easy task to find business who use one-time address.
But mixers won't be obsolete since there are scenario when people want to spend his Bitcoin from multiple UTXO/input where he'd lose his privacy if he simply make a transaction with those UTXO/input.
Mixer would help him if he make an transaction for each UTXO/input, even though Schnorr MuSig will solve this privacy problem.