#2. To test the waters for the future

Matt Riddle is a Superstar that any promotion would want. In 2018, he was among the hottest names on the independent scene and could have opted to work for virtually any promotion of his choosing.
He was dubbed as the future of the business and even WrestleVotes is now reporting that backstage officials on SmackDown have "high" hopes for Matt Riddle. As a result, we might see him have a slow build to the top, if the intention is for him to reach there.
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With Superstars like Matt Riddle, an instant monster push with the rocket strapped on their back simply won't work. The audience needs to get to know Matt Riddle as a character and get invested in him and his story in the long run.
Matt Riddle's move to SmackDown might be with the purpose of "testing the waters" and seeing what works and what doesn't. We anticipate that he's going to have a slow first year, but as long as he's regularly featured on television, he should be fine.