League: Like in most European sports. This rewards teams for consistency. A team that consistently beats opponents over the course of a season can surely be called deserved winners at the end of it, moreso than knockout play where luck can play a much bigger part. In terms of sporting legitimacy, the league simply offers a larger sample size from which to draw results. The problem with a league is actually a problem with the prevailing trend in esports since its inception; esports is based too much around one-off events with very few genuine leagues being run at all. It takes time, coordination and continuous effort to run a league, which most tournament organisers seem unwilling to invest.
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