The transition from 4-star to 5-star in PvP is difficult. Savvy players refrain from championing their 5-star heroes until they have enough 5-stars which are considered high tier PvP heroes. Many players who only champion one low tier 5-star PvP hero often regret their decision if they want to continue to PvP due to the increased difficulty in achieving a high rating and the teams they can't matched against.
Cause of the Issue
The root cause of the issue is matchmaking. Clearance Levels does not impact the matchmaker in respect to the number of 5-stars in the opponent’s team. Teams with no championed 5-stars are in the same ladder as a player who has championed 5-stars, but they are rarely matched if ever.
One potential solution is for Clearance Levels to restrict available heroes by number of stars. 5-star Clearance Levels would allow 5-star heroes and below, 4-star Clearance Levels would only allow 4-stars and below, and so on. Matchmaking would purely be determined by rating.
Part of the clearance level solution is that it would massively restrict the set of players you can fight against. At the moment, the game will match you against anyone in the same time slice (that restriction is in place so you can't attack people who have already completed the event and can't retaliate).
While that means high level CL9 players can attack you, it also means you can target people in lower clearance levels (probably the case when you skip through to easy matches). For some time slices and clearance levels, this might result in opponents completely drying up once people start shielding.