There is a problem, but the probability of it occurring is so rare that it can be ignored until a software solution is released. People are playing in the SoD and it's fine. Level up several heroes, share artifacts, what's the problem?! :D
The "new rule" you proposed would only complicate the game and cause headaches for players, especially against those who demand a time portal at the slightest provocation.
You've been explained almost everything. Are you ready to break the gameplay of EVERY game by forcing players to monitor the levels of heroes arriving in the tavern, playing on the verge of a time loss, just to somehow smooth out a situation that arises in 1 game out of 1000 (which may not even require it)? Doesn't it seem unreasonable? Moreover, you decided to do this on your own, even though there were no complaints or grievances from the participants.
Questions like:
- why can't I buy a support hero that I leveled up, but the opponent killed?
- what to do if there are leveled heroes in both slots? etc.
Also, the punishment for violation is somewhat vague. Like "well, I won't use a time portal here, just don't do anything extra," etc. I wrote an entire post about the good faith execution of the rules. Simply, the idea that "players are all reasonable, they should understand what is good and what is bad, they will figure out for themselves what is allowed and what is not" - doesn't work in modern realities.
And regarding the headache - players always check if a hero is leveled up before buying it in the late game, precisely to buy a dead main hero or a sub-hero. In this case, the trigger will simply work in the other direction - not "wow, what a great hero," but "we won't take this one."
What to do if there are leveled heroes in both slots... well, this is a one-in-a-million situation, so you can just increase the autosave frequency.
Why can't you revive a dead support hero? Because the very idea of reviving dead heroes is an abuse of the Heroes' mechanics. On regular maps, it's fine, but on a mirror map, it's a terrible imbalance that shouldn't exist.
I started discussing the procedural questions "what to do if you still buy" it, but by that time the discussion had already drowned in negativity, and no one was interested in such nuances.