Intros are pointless on an online platform for one simple reason:
The people who find your video based on the title or the thumbnail and don't know you as a creator don't give a s**t about who made the video or what the channel is called. They only care about the video that (apparently) has the content they are looking for. If they think you make cool content they will subscribe to your channel and don't need a constant reminder about who they're watching.
The only exception to this could be effective use of branding. But you need an actual brand to make that viable. For example, Rocket Jump has this short 5 second intro brand at the start of their videos, which is fine because their brand extends far beyond YouTube at this point. It promotes their company as a whole, not just their YouTube channel. A lot of YouTubers who currently use intros, however, do not actually have a brand beyond their own name. And that doesn't qualify as a brand in this context. Of course an intro could also just be part of a show, for example Shoddycast and their Storyteller series. But that's not the type of intro we're talking about.
In summary, use intro's only if you have a brand that is worth promoting beyond just your own name and your YouTube channel, such as a website or real-life shows or performances. And whatever you do, never make an intro longer than 5 seconds (doesn't apply to professional shows, series or movie 'shorts'). There are channels out there that have 30 (!!!!!) second intros in front of generic gaming footage or a vlog and I would smack those people in the face if I could. Nothing drives away viewers quicker. Not even unskipable ads.