This forum has a weird Nintendo bias, most places wouldn't even dream of putting Splatoon in the same ballpark as Bloodborne or Nioh. Nintendo haven't had a truly great new IP since Xenoblade.
Splatoon has outsold Horizon and blew it's attach rate away. If anything, there's a Sony bias.
Horizon sold 3.4 million units in two months. Splatoon sold 4.7 million in two years that includes holiday sales. With the rate at which Horizon is selling it would probably outsell Splatoon this year. As for attach rate, NawaiNey answer accurately sums up why Splatoon has a better attach rate.
Horizon Zero Dawn has been out for only a few months, It had sold 3.4 million in two months. Splatoon sold 1.5 million in it's first two months.
Also no one cares about attach ratio, it's more than evened out by the fact that Nintendo consoles have very few games to choose from in comparison to Playstation consoles. Look at the over all attach ratio of these consoles PS4 has an attach ratio of like 8 while Wii U had an attach ratio of less than 6.5, the sales on PS4 are jusy spread over way more games, while on nintendo it's usually the same few titles selling great while everything else gets ignored (usually cause everything else is beyond terrible). According to VGC there were only 21 games that sold a million or more units on Wii U, while there are already 98 on PS4 and will likely be like 300 million seller by the time this gen is over.
GTA SA had a massive attach rate between launch an a year after on a console that sold more than PS4. If a game is great enough and there is enough demand, the attach rate will bear that out. This argument that there are just a bunch of legendary games causing buyer fragmentation is ridiculous.
GTA SA had no competition on PS2. GTA 3 had revolutionised open world games and already popularised the series. Starting a new genre by itself was one of the reasons why GTA could garner so much sales. GTA SA was a sequel in the only major open world game series on PS2. There were no other good alternatives to GTA for open world gamers on PS2.
Horizon has competition like AC IV, Unity, Syndicate, Witcher 3, Skyrim, Fallout 4, GTA V, Batman Arkham City, Infamous Second Son and many more quality games in established franchises of the same genre all wanting the same customers money. Yes there are 60 million PS4 owners, but they also have like 15 great open world games to choose from, and they are unlikely to buy more than a few.
Also they may not even be in it for open world games as PS4 has plenty to play from other genres, while Splatoon is probably one of the reasons Wii U has as many owners as it does, not a lot, but a huge portion of them have it to play Splatoon which is another reason its attach rate is so high.
Horizon is the game of the generation so far for me.
I'm gonna throw in Rocket League, it's not a new IP but it may as well be. Played it every day going on almost 2 years, no game throughout my 26-odd years of gaming has ever caught me like that.
Rocket League is a new IP.