Hey Tanuj, I understand your problem. Watch Later Feed was
primarily built to only focus on finishing the Watch Later videos. You
can turn the extension on/off in the popup menu. You can click the power icon the top right corner of the popup whenever you're in the mood to browser youtube normally.
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Most
people have more than 100's of watch later videos, finishing most of
them will be time consuming just like you said. So, if subscriptions
page is accessible, most will feel like they're missing out on these
videos and they'll add the videos to watch later again.
You
wouldn't be wrong in saying that people will still add to watch later
even when the extension is turned off. But that's not something that
extension focuses on (currently), because my vision is: when the extension is turned on, only Watch Later videos must be shown in the feed.
Thanks for the feedback :)