This is a pretty high estimate and I wondered how it compared.
I want to look at Heated Rivalry and compare it with other fandoms I suspected of having a strong preference for a major pairing.
Here's some AO3 napkin maths for percentage of fanworks within a fandom tag featuring the most dominant pairing WITH otp: true selected. I am logged in, so it includes archive-locked works.
Heated Rivalry: 57% Shane Hollander/Ilya Rozanov
BBC Merlin: 33% Merlin/Arthur Pendragon
Wynonna Earp: 38% Waverly Earp/Nicole Haught
Supernatural: 25% Castiel/Dean Winchester
Yuri!!! on ICE: 39% Katsuki Yuuri/Victor Nikiforov
Pride and Prejudice: 27% Elizabeth Bennet/Fitzwilliam Darcy
魔道祖师 - 墨香铜臭 | Módào Zǔshī - Mòxiāng Tóngxiù: 29% Lan Zhan | Lan Wangji/Wei Ying | Wei Wuxian
Xena: Warrior Princess: 36% Gabrielle/Xena
Star Trek: The Original Series (this is TV, not reboot): 35% James T. Kirk/Spock
Stargate Atlantis: 37% Rodney McKay/John Sheppard
Rivers of London - Ben Aaronovitch: 20% Peter Grant/Thomas Nightingale
For shits and giggles, this is the same napkin maths done for a few fandoms where I did not expect to find a single really really dominant ship. I've taken the first ship listed under the relationships filter as the biggest.
Marvel: 9% James "Bucky" Barnes/Steve Rogers
僕のヒーローアカデミア | Boku no Hero Academia | My Hero Academia (Anime & Manga): 9% Bakugou Katsuki/Midoriya Izuku
The Lord of the Rings - J. R. R. Tolkien: 3% Frodo Baggins/Sam Gamgee
Transformers - All Media Types: 4% Megatron/Optimus Prime
Naruto (Anime & Manga): 8% Uchiha Sasuke/Uzumaki Naruto
Some other things to think about:
Using otp true means that a ship you feel is ubiquitous may nevertheless have a relatively lower percentage here. Like, what bullshit that lwj/wwx is only 29% when it feels like it's half the fandom tag, right? But, while it's tagged in 48% of fics in the MDZS tag, it's the only ship tag in only 29%.
While tagging ship is pretty consistent in general, I suspect that fandoms with plenty of imports from older archives might not be tagged so consistently, like, e.g. the K/S stuff.
The tagging habits of a fandom can also change a little, especially with regard to how secondary/additional romances are tagged. So otp: true might not necessarily mean precisely the same kind of focused fic content in every fandom owing to that factor. 🤔
Some of these fandoms are big and some are not as big. I know measuring as a percentage should account for that. However, in smaller fandoms one (1) REALLY obsessed shipper can swing the percentage. So if this were to be a proxy for how a bunch of people in a fandom are behaving, it might not be so useful in, for example, Rivers of London, where 276 fics make up that 20%.
My conclusion is that the Heated Rivalry AO3 tag does seem to have an uncommonly large proportion of works featuring a single dominant pairing when otp: true is selected. My hunch is that it's relatedly newer and might diversify over time. (ノ◕ヮ◕)ノ*.✧