There is definitely a Rule of Diminishing return just as there is the Sunk Cost Fallacy. I suspect that the Venn diagram of those two circles would be close to one circle in some cases for certain audiophiles. For example, someone subconsciously suffering from the Sunk Cost Fallacy has spent years and enormous amounts of money in their pursuit of perfection and yet never attain it as perfection does not exist. So each subsequent purchase is rationalized as moving closer to audio perfection. Yet the reality is they would not / could not admit - even with hard facts - that the recently purchased $100K speakers are marginally better or equal to the $10K speakers or even the $5K speakers because they suffer from the Sunk Cost Fallacy. The diminishing returns set in long ago for each dollar spent.
The Rule of Diminishing Return applies to many industries where the marginal performance gains do not justify the cost, yet at the high-end market of consumable goods, the real reason is justification for many that they can exhibit how they can "afford" to pay $150K for certain speakers, or $200K for a vintage Jaguar, or any luxury item for that matter. Some may actually know they're getting no real qualitative improvement for their investment because the point is to exhibit their wealth.