Then they wouldn't be allowed to rejoin.
There would be no reason to delete a replay besides hiding sensitive information mentioned in team chat (in that case, try to prevent team chat from getting recorded if possible).
Addressing the uploading concern, it should really only take a couple seconds (the amount of time it takes to queue up and enter another clan war).
I'm pretty sure 90 percent of the most active players wouldn't mind this system. It would, in fact, bring back a lot of players.
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Or have a bot that records the clan war on its own, not from the player's perspective. That would somewhat kill the main purpose though.