Mercedes dominated the 2020 season by winning 13 out of 17 races, including 11 victories for world champion Lewis Hamilton.
Red Bull finished a distant second with two wins for Max Verstappen, but caught up in 2021 which led to the most dramatic title showdown in recent memory between the two.
Red Bull won all but one race in 2023 as Verstappen romped home to his third consecutive world title. Meanwhile, the team switched its development focus to 2024 in the summer, which has led to pessimism from rival teams that it can be caught next year.
When asked why he thinks anyone can catch Red Bull next year, Horner replied: "We saw that in 2020 with Mercedes. If you remember, that was their most dominant year ever and yet, we were able to beat them in '21.
"There is a subtle rule change but I think that nothing stands still. We've seen competitors coming closer at different venues and I'm sure concepts will converge.
"Stable regulations always concertina. I don't think we'll ever be able to repeat the season that we've had, but hopefully, we can take the lessons from RB19 and apply them to [RB]20 and come up with a car that we can defend these titles with."
In reality, 2024's regulations are more stable than they were between 2020 and 2021, when a rule change aimed at cutting downforce led to significant redesigns of the floor.
Verstappen won the season-closing Abu Dhabi Grand Prix by 17 seconds, a larger gap than his 11-second margin in the first race of the season in Bahrain. But Horner believes that rivals teams Ferrari and Mercedes having significant ups and downs has contributed to the larger gulf.
"It's been different people at different times. It's swung around from McLaren having a surge, Ferrari having a surge, Mercedes turning up at one point, so it's moved around from venue to venue," he said.
When asked if there was any hint of annoyance that a Singapore slump prevented Red Bull from claiming a clean sweep of 22 race wins, Horner replied: "No. It leaves you humbled that there is still something to strive for and it's a useful lesson that things can change quickly.
"Singapore was a standout weekend. You guys have been asking me since about race three, 'Do you think you can win all the races this year?'.
"To win 21 out of the 22 races is insanity. For Max to have led over 1000 laps, for him to have won 19 races, to have broken McLaren's record from '88, to have broken Seb's [Vettel] record from 2013 – the win ratios, all the percentages that he's hit...
"This car will go down in history certainly for a considerable period of time as the most successful car in Formula 1 history."
