Did they ask if there was a chance on another carrier? I don't think Delta is obligated to offer that as an option.

I don't think a trans. voucher is reasonable. I just had a trip delayed due to a mechanical. It was a Skymiles trip and I asked if they could credit me miles for the delayed segments. Not only did they do that, but they changed my booking class to Y which scored me a few upgrades. Had I not asked, I doubt they would have offered anything as compensation. All that was a nice gesture, but they were under no obligation to do anything but get me to my destination in the timeliest manner possible, which they did. Planes break down sometimes and I'd rather wait a little for properly working equipment.