Some people are losing it. Let's see if we can make some sense of it all.
Install base argument favouring Horizon sales over Zelda is a fair assumption but we must also remember the 'low' Zelda sales is caused by 'low' Switch sales. For instance, Shadowfall sold twice the amount of Zelda in their launch weeks. So we can't really blame it being a launch game especially when it's one of Nintendo's biggest exclusives that's expected to push consoles. So now we have to answer if Zelda failed to push Switch or if Switch failed to push Zelda. Buuuut, we then have to apply context. PS4 launched in an very busy month of November while Switch launched in a relatively much weaker month. Personally I think both Switch and Zelda sales are Ok but lower than I expected.
These are only a few days of sales so we shouldn't be predicting failures. I also think Zelda will have better legs than Horizon.
