It's hard to know how much Cartoon Network Street was a cause of their problems, and how much it's a symptom. Ben 10 probably isn't as good a brand as Thomas, and Thomas has the advantage that many parents also feel nostalgia for it. At the same time Thomasland was a much bigger investment than the Ben 10 coaster. It's not like Icon, where you could argue that it should have had a big impact. I know the queue was quite well themed, but it was clearly built on quite a tight budget. Any atmosphere built up in the queue melted away as soon as you got to the station, and the outside of the ride was a lot of corrugated sheds (I have no idea if this resembles something in the TV show).
I'm not saying they should have spent more on it. Presumably they couldn't. But Ben 10 was clearly a step down from the previous 'big investments' like Storm Force 10, Excalibur, G Force etc. For that kind of budget they were never going to get a big impact ride.