Good dapps and real users are kind of the holy grail when it comes to blockchain platforms, and Steem has a good amount of both and was available to buy at probably a relatively low price.

That is a very expensive acquisition to kill most of it off - isn't it?

Quite the contrary I think. Let's say they bought Steemit for $20M (this is just a made up number, I don't know what the price was) and they have maybe a $400M stake in Tron (also a made up number), then if TRX goes up even just 5% as a result of all this then it's well worth it for them.