The selling numbers of any game are often tied to the success of the previous one from that company/franchise, not so much its own. See Starfield...
Nah. I think Valhalla sold well because it was different than Origins & Odyssey, at least on the surface. It was the first ac game in half a decade to not be set in a boring fucking desert. Plus, the series had never done Vikings before, and Vikings are exciting, new, and cool.
So what do they learn from that? They decide to go back to the desert and set the whole game in a city full of tiny, copy-pasta'd-looking, beige mud huts that are barely indistinguishable from the beige desert. Yes, ac1 did that and it was massively successful, but ac1 did that partially because ac1 was a shitty tech demo where the devs were trying out the basic gameplay systems and engine without wanting to do something as complicated as Renaissance Italy yet. The setting itself was really dull, though (in fact, apart from the central engine and parkour mechanics, ac1 is a pretty terrible, repetitive, and half-baked game all around).
Also, the people yearning for a "back to basics" like the original ac games were super complicated are idiots. Their combat was always just rock/paper/scissors and had almost no strategy. You could also just hold down the dodge button in crowds and dodge endlessly. The stealth was always overly simplistic and any difficulty involved was mostly just annoyances of waiting long enough for someone to move slightly to the left. All of the original ac games had terrible gameplay mechanics. However, no one noticed or cared because they were usually really fucking gorgeous looking (for their time) and there was usually a bunch of shit to do in them.
Origins' combat wasn't good, in fact it was so basic that it was kind of dull, but it was also the first game in the series to have combat that made sense mechanically. In the other games, you'd just kind of hold down the buttons and see what happened. Or just switch weapons to instantly overpower people. Or just beat on someone while they block endlessly until, randomly, they suddenly didn't block anymore, and you'd insta-kill them, but never be quite certain why.
These games have always been mindless and stupid excuses to just run around gorgeous looking historical settings.