Yeah but we know 4 and 5 sold way less than the original 3. MS didn't give official numbers but we can use NPD and weekly numbers that we do get and determine 4 and 5 sold significantly less. Splatoon on the other hand, sold more than double the best selling 360 Gears game.
We don't actually know that Halo 4 and 5 sold way less than the original 3 actually. I'd say it's quite safe to assume they sold less than Halo 3, which I'd see as the series peak in regards to popularity, but Halo 4 for example was the fastest selling Halo game out of the gate (including Halo 3).
Exciting news for the Halo team at 343 Industries and Microsoft Studios – critically acclaimed, "Halo 4" is now the best-selling Microsoft Studios title ever in the U.S. market*, surpassing its predecessor, "Halo 3" for sales during each respective launch year.
Now, I think it's safe to say that it's long-term tail wouldn't be as strong as Halo 3, because the MP wasn't as well received and Call of Duty ruled the world by then. But people do have a tendency to massively lowball Halo post-343i simply because it's no longer the top dog. Halo 5 was also reported as having sold in line with previous series entries... though the market shift to digital, and it being release prior to NPD factoring in digital sales makes it a lot less clear where it would've stood. The series sales didn't crater in the manner that people tend to imply however. Halo 5 was reportedly over 5m within 3 months from launch. There are just games that are much bigger these days.
For Splatoon 2 I'm looking at its sales. 13 million bought this thing, simple. For Halo we've got nothing concrete besides that 20 million downloads thing cause apparently the exact numbers we have of it clearly not being a success on Steam past launch in its first season aren't supposed to count cause apparently turns out being big on Steam wasn't that important. What would you look at to say that clearly Infinite is a bigger success?
It's not about it being F2P. It's about Halo Infinite having nothing officially confirmed that says "this game is bigger than that game 13.3 million people bought at or close to full-price". We don't know how much it has made, and "20 million" downloads isn't big enough to be more impressive than what Splatoon 2 did and I'd think that would be obvious.
There is alternatively nothing that says that Splatoon is bigger by having sold 13m copies in 5 years than a game that was played (not downloaded... played) by over 20 million players within the first 2 months. You're acting as though everyone that bought Splatoon 2 played it actively. I own Splatoon 2. I bought it at launch, as I had few titles for my Switch at the time, and had recently been lttp on the original game. I've played it for maybe 5 hours total in all those years. A sale is a sale, but if we're talking player retention, there is very little reason to believe Splatoon is currently played by more people than Halo. Hell, I'm not even sure how strong a case there would be for that pre-Infinite due to MCC + 5.
There is basically no question that Halo 4 sold over 13m within the same window of time as Splatoon 2, and that was the Halo game with the absolute lowest player retention in the series. It's nowhere near the measure of popularity you're arguing it to be.
In regards to how much Halo has made Halo 4 reportedly brought in $300m in its opening week. If we were to assume all 13m copies of Splatoon 2 were bought for $60, with none being bought at a reduced price, or being bundled with the Switch (which happened quite heavily, so I'm not sure why you tried to level this against UC4...), you'd arrive at $780m in raw revenue from sales... in 5 years, vs Halo 4's $300m opening week. Halo 5 despite lower sales was reported to be the Halo game with comfortably the highest revenue, as a result of MTX with stuff like REQs for Warzone.
As mentioned before, Halo isn't top dog anymore, but that's largely down to just how insanely large some games have become in recent years as the market has expanded. Back in Halo 2's days, 13m puts you amongst the biggest games in existence. Today Borderlands does that with its eyes closed and Elden Ring manages it in a month. The raw sales of Splatoon are impressive, but nowhere near to the extent you're painting them to be, and is precisely why a historical juggernaut like Halo is making a move towards f2p (along with even the current genre king CoD)... because there's simply more money to be made there.
People can say in clear terms "Uncharted 4 is a bigger game and a bigger success than Splatoon 2" because you can simply point to it having made $1B, which Splatoon 2 clearly has not. Similarly, you wouldn't see me being unconvinced that F2P Fortnite is bigger than Splatoon 2 cause the game made like $9B in 2-3 years. Halo doesn't have that. It doesn't have anything you can clearly relate to Splatoon 2 and say "oh this is a bigger success than Splatoon 2". On its own terms, separate from this conversation of 1st party shooters, Halo Infinite doesn't even seem like it was a big success past the first two months.
I'm not even sure any of the past Halo games sold much better than Splatoon 2. The series was at 81 million sales total as of Feb. 2021. Even if we chose to pretend the re-releases and spin-offs didn't exist and just divided that number by the 6 mainline FPSes (H1-5 and Reach), we'd still get an average of 13.5M per game, which Splatoon 2 would pass pretty soon.
Splatoon 2 has nothing you can cite that points to it being clearly bigger than Halo either. If anything there is only any evidence pointing towards the opposite (Halo being played by more players). I think you'd probably be very hard pressed to find anything that indicates that Splatoon makes more money than Halo, because at the end of the day, Splatoon isn't a game design around its live service revenue, which multiple publishers have cited in recent years as being far more lucrative than initial sales, hence why the model is becoming the new norm amongst gaming's biggest IP.