"Do you want a league where your team is a group of all-stars and your opponents are all unmanaged teams? That seems to me the end goal of the policy you suggest."

These are the only two alternatives ? That is a fallacious argument. I try to make each one of my teams a team of all-stars and a staff full of aces. I do this by drafting the best I can and trading to refine my team and make it better. If the trades are fair on an individual basis at the time of the trade (no looking back X years later and pointing out trades that were fair but now, through injury or development now is imbalanced) then they should always be allowed. I will not be able to amass this team of stars because everyone else is trying to do it as well and there are many smart people in PB. They, the other GMs, through their own abilities, should be the ones to stop me, not new, restrictive rules.
If someone does get an all-star team then I applaud them. If they can maintain it by fair trading and shrewd drafting then I admire their skill. It's tough to do. I never want fair trades overturned to kneecap the sucessful teams.

"You help me in league A and I'll help you in league B, or browbeat the opponent into a bad trade, or take advantage of a newbie who doesn't yet understand the nuances of PB, or run a farm team."

In the 10 years I've been in this I've only seen one situation that appeared to be suspicious. If you believe otherwise, then show some facts. Otherwise, let's not spend a lot of time and energy on new rules to counter a problem that doesn't exist. As for molesting newbies we already have rules in place to prevent that.