While obviously they'd restore adult content to Tumblr, my guess is they'd keep Tumblr as a somewhat mainstream brand, and not let it be too directly associated with their existing properties to maintain mainstream appeal. A YouTube competitor would fall along the same lines.
Although we all know a few people here and there who don't know YouTube is owned by Google, the majority of people do understand the YouTube + Google/Alphabet relationship.
I think this is very tech centric view. I asked my wife (tech savy enough but doesn't work in the industry) and she didn't have any idea who owned Youtube, and had never heard of Alphabet.
I know it's only one data point, but I think it's easy to be blinkered when actually the truth is probably that most people outside of tech have no idea about tech company relationships.