Name one decent Disney animated movie that was released after Pixar was founded (excluding Pixar films post-acquisition).
There aren’t many because most of Disney’s talent left around when Pixar was founded. Without movies to continuously hook new kids, their entire empire is a house of cards.
Pixar was founded in 1986. They started releasing theatrical films with Toy Story in 1995. Are you really saying there hasn't been a single good Disney animated movie in the past 25 years? What's the basis for that statement?
The history of the two studios is way more involved than what you're suggesting, and talent moves around between studios quite a lot. And 25 years is a long time; there have been entire generations of talent that have risen up since then, and trust me, they're not all at Pixar.
Source: I worked at Disney Animation for nearly a decade (and closely with Pixar during that time). I'm certainly not unbiased but probably way more informed.
The argument is that HBR's article is just speculation, nothing more.
Some mergers have literally saved companies, some have ruined them, some are considered 'unsuccessful' for esoteric reasons and others successful for purely numeric ones.
There aren’t many because most of Disney’s talent left around when Pixar was founded. Without movies to continuously hook new kids, their entire empire is a house of cards.