And how do you explain X/Y selling more than Sun/Moon despite having half the install base? 

The more people own a system the more potential buyers, that’s general logic. To call S/S a mediocre opening while ignoring install base is ignorant imo. 

Half the install base? The 3DS had around 14m in Japan when X/Y released, compared to 10m of S/S. 

Was this game also more expensive compared to the other ones?

What is strawman ? 

It still invalidates your argument. it sold more on a smaller install base

You don't understand my argument then..!

Plus: 14m 3Ds install base is LARGER, not smaller than the Switch’s 10m. Can you not count? 

He's asking how you can explain X/Y selling more than Sun/Moon even though X/Y released on a much smaller install base than Sun/Moon did? By your first point, Sun/Moon should have sold much more because when those games came out, it was on a significantly bigger install base and selling to more potential buyers. Especially when you consider when those games launched, hot off the heels of Pokemon GO and its worldwide takeover bringing Pokemon close to the levels of popularity and awareness it was back in its peak in the late 90s. 

You can't just multiply the Switch's install base and apply the same to Sword/Shield thinking it will sell that much more. It doesn't work that way and it never has. Every Nintendo handheld that has had 2 mainline Pokemon games, the 1st one always outsold the 2nd one.
Game Boy/Color: Red/Blue > Gold/Silver
Nintendo DS: Diamond/Pearl > Black/White
Nintendo 3DS: X/Y > Sun/Moon

Bigger install base be damned. Because a portion of the audience are usually satisfied with just the 1st set of games. Add to that, the remakes of a prior generation or enhanced versions Game Freak releases in between generations, by the time the 2nd generation comes out, those people who skip out on it do so because they've already had their fill with Pokemon on that system and are ready for the next. And this also goes for many of Nintendo's other franchises, not just Pokemon.
Mario Galaxy 1 > Mario Galaxy 2
Ocarina of Time > Majora's Mask
Twilight Princess > Skyward Sword
DKC 1 > DKC 2 > DKC 3
Wii Sports > Wii Sports Resort
Metroid Prime > Metroid Prime 2
I could go on and on.

Most of the time, a bigger install base did not equate to better sales for the next game.

Think man, you need to quit the spin.

First of all, Sword and Shield's figures do not account for Amazon, digital and vouchers. When you take this into mind, the figures are closer to Sun and Moon's 1.9m figure. Secondly, this isn't the lowest opening. That is Ruby and Sapphire. Which opened at 1.2m. diamond and pearl also opened at 1.5m, which this game has already passed as well. This game also costs more than any other Pokemon game and it is the biggest launch on the switch and any current generation platform. This game is a success, and there is no way you can spin it otherwise

First Week Sales (Famistu)

Pokemon Black & White - 2.64m
Pokemon X & Y - 2.1m
Pokemon Sun & Moon - 1.9m
Pokemon Black & White 2 - 1.62m
Pokemon Diamond & Pearl - 1.59m
Pokemon Omega Ruby & Alpha Sapphire - 1.53m
Pokemon Gold & Silver - 1.43m
Pokemon Heart Gold & Soul Silver - 1.41m
Pokemon Sword & Shield - 1.36m
Pokemon Ruby & Sapphire - 1.25m
Pokemon Ultra Sun & Ultra Moon - 1.2m
Pokemon Platinum - 0.97m
Pokemon Fire Red / Leaf Green - 0.96m
Pokemon Yellow - 0.85m
Pokemon Crystal - 0.71m
Pokemon Let's Go - 0.66m
Pokemon Emerald - 0.64m
Pokemon Red/Green/Blue - 0.14m

So other than Pokemon Ruby & Sapphire it's the worst physical opening for a new gen. Including Digital it's maybe 4th, behind the last 3. A definite decline. We're a long way from where we were 2 years ago when everyone was claiming the first console pokemon game would be an unstoppable god. Switch has done wonders for elevating Nintendo game sales, but that's not passed over to pokemon.

Of course these sales are still great, but they should have been better. Just like the game.

At a glance it's seems like lukewarm sales for a new gen Pokemon. However, plot twist; digital ends up being 50%, so it actually sold 2.7M.