While it is nice too shoot for 50% win rate

A hero that just came out should not have a 50% win rate.

Here is why I think this:

If people don't know how to properly play against the hero, it's win rate should be inflated. Then after a week or so we can expect to see more counter play discovered and thus a lower win rate for the hero.

However this also works the opposite way, people have not fully discovered possible synergies with the hero and these synergies could improve the hero's winrate.

In my experience with most mobas (HOTS being no exception)synergies usually have less power in determining winrate when being compared to counter-play, meaning winrate will go down more(due to couterplay discovered) than it will go up(due to synergies discovered).

In conclusion I suspect Zul'Jin's winrate will indeed drop into mid- low 40s after a week or two.

Now of course if changes are planned pre hero release shooting for 50% is fine at release, because a follow up patch to maintain a 50% winrate after couterplay and synergies are discovered is healthy if patched in a timely fashion.