Not resetting the trophies will keep the top players at the top and will make them not put much efforts/money. Since reset put everyone at same level, others get a hope that they too can now get past the top players by upgrading their team/buying stuffs.
Ideally every tournament begins with a clean slate. People start from scratch, go through qualifiers, followed by elimination and so on till finals. But based on last tournament, we know how it went when everyone above 4K was reset to same level. That’s where I guess they took this intermediary approach so atleast the ones around 4K do not get paired with the 5K+.
Edit: I understand the argument of not wanting to participate in tournament due to influx of higher level players coming down, having a separate battle section for tournament. But guess that has it’s own share of problems like separate management, split of player base reducing participation etc.