No, you can concatenate video segments with basically zero cost if the segments are encoded using the same encoding parameters and afaik youtube already re-encodes user videos.

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dandellion 4 days ago | parent [-]

What's to prevent ad-blockers from just skipping ahead when there's an ad playing?

	▲	jack_pp 4 days ago \| parent [-]
		Simply refuse to send original stream packets / ignore seek requests by the client until the ad duration has passed. Sure you could pause / mute / alt tab for that duration but the point is being annoying so people buy premium if they hate ads so much.