Okay, I've been lurking for quite some time but when I saw this I had to come out of the shadows.
Let me start this off by telling you a little about myself. I worked in wood coaster maintenance for about 15 years for another park chain in the country. Since then I have gone on to a regular "desk job" in engineering. I have worked with some American roller coaster manufacturers consulting for different aspects - mainly doing calculations for certain track elements and safety mechanisms. I've never designed a roller coaster in my life... I don't want to give you the wrong impression about what I do, but I can speak with a lot of credibility on the issue.
With that being said, your statement that "poor upkeep" being the reason why some rides are rough and others are not is relatively unfounded. There are numerous reasons why a ride could be rough or smooth. It could be anything from initial design to the weather that day.
If you saw someone working on the track on the morning of opening day at Holiday World that's great, but you would see it at Kings Island, Six Flags and anywhere else. They say that when you build a wooden roller coaster you're building a marriage. Those things are maintained every day of the season. They don't just take the day off because it's closing day. It still has to cycle hundreds of times that day. You're not going to risk damage to the trains or the rest of the track just because the park is closed the next day.
As far as The Voyage is concerned, I've never ridden it. I can tell you that the steel structure probably messes with the physics. It could be a good thing or a bad thing, but I haven't really seem an overwhelming preference for it. According to most accounts, The Voyage was built for under $9 million. That's extraordinarily inexpensive for a ride that size. I just think that they tried to do too much on too little. You shouldn't have to explore options for new trains with a ride having been open less than 5 years. What happened to those trains anyway? I imagine that they determined that The Voyage would have torn them to pieces with regular use. I don't think the Gravity Group is really responsible for any of this. They're good people, they make a fine product. If I had to submit a guess I would say that they were asked to build this ride on a certain budget and they did what they asked... and Holiday World got what they paid for.