If a film is lucky (aka lucrative) enough to spawn a series, the law of averages dictates at least one sequel will suck. And not just suck, but suck really, really bad. It may even be so poorly regarded that afterwards the franchise will semi-reboot, with the next movie ignoring it completely.
Thanks to rising budgets and falling audience numbers, studios are increasingly relying on long-term franchises to keep themselves afloat. Practically every big budget movie now has to have the option for at least two sequels, and maybe a prequel and a spinoff, too.
