FWIW, while speakers are the single most important component in your system, I can't agree that diminishing returns is relevant, but 'relevance' can only be determined until after you have 1) Developed listening skills, and 2) Determined what sound you like and how you can obtain it. We all seem to chase 'better' speakers, yet in our early years really don't know what 'better' is and we know that. So we search. When we find that speaker and then try to improve on it we see the rule of diminishing returns set in as well as a high degree of frustration. :-)
"I'm a believer"
I’ve been around high end audio for a great number of years. I have had the opportunity to hear, at shows, at audiophile friends homes and at audio shops, a great number of high end speakers: old and new, from the low, to the ultra megabuck price ranges. I’ve heard very, very expensive speakers that didn’t sound so good to me, and then, I’ve heard vintage speakers or relatively affordable speakers that just knock my sock off. In all my personal experience in this great hobby of ours, IMHO, there is no other item in high end audio that fall under the "Rule of Diminishing Returns" like loudspeakers.