Isn't that mostly due to stagnation of resolution? 1080p is still the most common display type and has been around for over a decade and with increasing GPU prices it is here to stay for probably another decade.
To some degree you can adjust for resolution since even back when SLI/Crossfire were popular, we still had a similar situation to today. However instead of 1080p, 1440p, and 4K being the primary resolutions you would see a shift where 1080p (or the 16:10 equivalent since that was more common) was the middle resolution.
Here's some of the benchmark results from the
AT review of the 4870 which shows why SLI was necessary. Even at the middle resolution of 1920 x 1200 the top cards struggled to hit would be considered a minimum acceptable frame rate today.
That's just for games like Oblivion which weren't the most demanding titles. If you look at the results they have for Crysis then 1920x1200 is the top resolution that's benchmarked.
We don't have a situation where a top-end card like a 3080 or a 6800 XT needs SLI/Crossfire because it can't even manage to hit 60 FPS in a resolution like 1440p in many titles. There aren't even a lot of games where this is true in 4K. There are even a few games that these cards will run well enough that most people are unlikely to have a monitor capable of displaying all of the frame since the refresh rate isn't high enough or would often require a monitor that's even more expensive than that top-end GPU.
And there were even cases where SLI/Crossfire still wasn't good enough. Some games were just that demanding. Here's results from
a different AT review showing off Metro 2033 results in 1920x1200 where outside of the most powerful GPUs, it's just not possible to hit 60 FPS even when using an SLI or Crossfire setup. Move up to 2560x1600 and even a pair of 580s in SLI are only going to get 35.5 FPS. Those cards retailed for $500 when they came out, which is about $650 today when adjusting for inflation.
So in a decade we've gone from where the effective maximum resolution for many games, even when running SLI/Crossfire, has become the lowest resolution and there's a top-end resolution (4K) is effectively 4x the pixels as that. The MSRP for a 3080 wasn't too far off of what a 580 would cost in today's dollars and that single card delivers more than acceptable performance in that high-end resolution.