Disclaimer: Not a Wordpress expert. I don't know much about Drupal or Joomla. But what I noticed was back in the early 2000's PHP was the craze; a simple language for the web, it was those one-click LAMP/XAMP installers. All in all it was just a bunch of small circumstances drawn together that snowballed into enough momentum for the mainstream spotlight.

A ton of budding 'techpreneurs' whose $family_relative_or_friend 'knows PHP, therefore can wrangle Wordpress' could get them a site up and running when they 'come over to their house for some snacks and chips over a weekend'

"Why aren't there any contenders?" Those entrepreneurs are now successful businessmen who have enough budget to keep being on Wordpress, because 'it just worked'. That battlecry spreads like wildfire for a large amount of time, hence "production-ready/polished" comes into mind.

Moreover, depending on the businessman, it's not much about 'the synergy between JavaScript on the front and back end' or other technical detaiils. It's more of the can I get my $son to do that for me on a saturday afternoon given the following X things I need.

"Clearly WordPress thrives on its massive community, and maybe its just a matter of time until a worthy opponent emerges to gain the necessary momentum." Yes, empires rise and fall. It won't be sudden, like any piece of software, Wordpress will also slowly lose relevance in the course of time depending on the amount of people working on it.