I feel sad for ownCloud and Nextcloud. ownCloud has a brand and this fork dilutes it. It will be on everyone's mind especially since the founder has left to create a competitor. For Nextcloud, unless they can explicitly state why this fork exists, it's hard to side with them. What kind of ideological differences are we talking about? Sure, you don't need to throw around personal accusations or finger point but it would help to know what motivated this.

On a related note, this is a risk which many platform companies take. If you opensource your project completely, then anyone can fork it and take it ahead if the forker is a big or well known person. I think we got really lucky with iojs and node.