Yea, but they could invest in new content aside from 21st Century Fox... Netflix didn't build up their original content via acquisitions, they invested in content creation... Disney could do similar, especially with the various IPs they already control.
Sure, perhaps there's a difference to be drawn between "is in competition" and "is competitive".
Disney is currently able to _try_ to compete, but their new streaming service is not competitive due to a lack of content, and won't be for years if they try to build up their portfolio organically as Netflix did.
If they bought 21st Century Fox, then arguably they would be immediately competitive with Netflix.
This is a classic build vs. buy decision, where (regulatory concerns notwithstanding) it's often better to buy as a well-funded new entrant that's trying to catch up with an incumbent.
