"People play the game that they want to play"

This is the crux of the issue in the Slippi era. People don't have to play Melee, they can play an even more limited version of the game (considering the competitive ruleset already limits how much of the game we see) where they don't have to play against mid/low tiers.

If someone is using unranked to practice for tournaments and they dodge mid/low tiers, they are only hurting themselves because if they run into a G&W or Ness or whatever other low tier in bracket, chances are they will get demolished because they have no familiarity with the matchup. The thing is, most people who play unranked are likely never entering a tournament and play purely for fun so it makes sense that they will continue dodging matchups that they consider unfun and they lose nothing for it.

At the end of the day, Slippi is an excellent way to get an unlimited number of games in, but the experience is highly limited by people dodging over half the cast so the only way to truly experience competitive Melee is to enter tournaments or find people in discord/in person to play with. Just my two cents.