Personally I prefer lower case or the camelCase (not PascalCase). But there is an ugly but important point I want to raise is the ease of adopting for new comers. Laravel took a lot of the focuses these years and they used PascalCase. If Yii use PascalCase and when people are sick of Laravel or they simply just want to explore what’s available then Yii will have the edge over those not using because these people will find Yii more familiar.
I know people don’t switch over just because of namespace naming convention and I quite hate to say one framework has to live under the shadow of another framework but small things like this will build up to it’s ‘friendliness’ to the users.
In the end it is namespace we are talking about. It should be relatively easy just to refactor the namespace path during an upgrade and people are expecting changes from Yii 2 to Yii 3 anyway.