I don't believe Customer Service has any knowledge of what you're talking about. They're not developers and afaik have no hand in hero design.
Most promotions heroes lose enchants in certain stats and gain enchant potential in something else. Just because the items are different. With key heroes Expertise is one to watch. There are points were promoting a hero is a bad choice, depending on your strategy and how you play, or use your heroes.
Re Lifesteal in specific, there's an interesting choice there to consider. In my personal opinion there isn't much difference between 200LS and 500LS. Others may have a different opinion. If I get in a situation where 200LS doesn't keep me alive, then I don't use those heroes in that situation again. Also, you can't look at Lifesteal in isolation, but you must consider damage output as well. If a hero does twice the damage with half as much LS the healing is the same, but the dmg is 2x. That's straight up good. More likely a hero may gain say 20-30% extra damage in exchange for half it's LS. Now you have to decide if that's a worthwhile trade, I would take it. Cyan+1 adds alot of HP to many characters. This gives a bit more "elbow room" for LS to work as well.
It's good to not just blindly promote every hero as soon as you're allowed. Understanding the full consequences is a good thing. There may be ways to change your gameplay to capitalize on the new stats. Or there may be certain heroes you want to continue working as-is and thus should not promote. Welcome to advanced Dragonsoul strategy.