Which units would you replace in the Sylvan castle?

You are absolutely right...
The ability to deal both physical and magical damage at range is a great skill.
As for druids specifically, the lightning damage doesn't directly depend on their number, but rather on a certain decreasing coefficient, so just divide them based on how many can fit in a hero.
It's also very useful to separate one to cast "stone skin" on your own units or to transfer mana to a hero.
So, the druid definitely deserves its place.