The article specifically argues that they should do this in order to further grow their customer base, and retain customers that are unwilling to pay current prices.

Why couldn’t they keep existing subscribers ad-free? Those people are currently paying huge subscription fees.

Why would it be impossible for Netflix to implement advertising in the way this article suggests?

Of course it's not impossible, they could do it if they wanted to, but that's not what would happen. That's not how anyone does it. Inevitably they would add ads at approximately the current price, and raise the price of the ad-free tier.

Spotify seems to have been successfully doing it as described for many years now, no?