After being acquired by Google, YouTube got better for years (before getting worse again). Android really improved for a decade or so after getting acquired by Google.
The Next/Apple merger made the merged company way better. Apple probably wouldn’t have survived much longer without Next.
I’d argue the Pixar acquisition was still good for a few decades after, and probably made Disney better.
A good merger tends to be forgotten, where the two different parts work together seamlessly to the point that people forget they used to be separately run.