Hi guys, I'm new here, but I follow Carowinds as well as the general amusement park industry pretty closely, so I thought that I would add my knowledge...
Thunder Road was a great coaster, but it had fallen into disrepair (on the part of multiple owners, including Cedar Fair) and would have needed a large amount of money to bring back to better condition. We should keep in mind, however, that Cedar Fair did rebuild portions of it to keep it running for a few more years. They planned to expand Boomerang Bay anyway, and it wouldn't have made sense to dismantle Thunder Road after the expansion, because then it would be in the middle of a waterpark and less accessible. Also, it was 39 years old. It wasn't a huge draw for crowds. A massively expanded and re-themed waterpark is, however, and it makes sense that they chose the waterpark, because in the long-term, that will be a better buy than an already 39 year-old coaster that won't hang around forever. I miss Thunder Road, but I understand the reasons for its removal, and I trust that Cedar Fair has great things in store for the park.
Also, I'm fairly certain that no one here has noticed that the park is also converting one half of the old Action Theater into an interactive Plants Vs. Zombies experience: https://www.carowinds.com/what-s-new/plants-vs-zombies/?mobile=0
And if that and Carolina Harbor aren't enough, there is still another ride for 2016 yet to be announced that will be a great and unique addition for the park.
Cedar Fair plans to continue heavily investing in Carowinds, which makes me very excited for what will come, even if there are prices to pay, such as the removal of an iconic coaster.
Cheers,
Merthecat