I've wondered about this, having never heard gear better than my own.
A long time ago, I used to think it kicked in after about £2k per box (speaking generally), but I’m not so sure. I’ve heard some stunning stuff like £10k Class amplifiers that sound as good as you could imagine, and I’d be happy with as end-game. Hearing some great sounding speakers in the £15-20k price bracket, which I always thought would be hard to better without spending six figures, have been obliterated by speakers at twice that price. Where that value lies is down to the individual, but my perception of ‘law of diminishing returns’ has shifted.
If someone has a system based around £200 per box hears a system based on £2,000 per box, they will either love what the more expensive system is doing, or not feel it is worth the extra - this then becomes their “LoDR”. We’re all different, so our LoDR will be different to everyone else’s. And some people hear others say it about certain equipment, so it becomes fact that that is the case, and they’ll repeat it on forums - because many on social media that don’t really know what they’re talking about presumes that everyone else on social media (in the relevant hobby groups) are experts.