I mentioned a lot of this in the other thread, but Elitch was pretty crowded when I went last summer and that was with several attractions down for the count. While it is true that Denver has a lot of competing attractions including simply being outdoors, it's still an area full of young people and young families and thrill rides can be a huge draw if done correctly.
The problem is that Elitch was just a bad park... Just not a great selection of rides, not a lot of space, and obviously they didn't really control the land so there wasn't a whole lot of reason to heavily invest in the place to make it into a destination. A new park done well could be a completely different story.
I'm picturing a Toverland type park. Big indoor area for partial year round operations, then some high-profile outdoor coasters and big rides for the main season.
Lakeside is it's own can of worms... the park is awesome in it's own right, and it also drew a nice crowd when I was there, but it was definitely a bit rag-tag. Looked like they had a ton of projects going throughout the park to fix things up and add a few rides, but it also seemed that everything was being done with a skeleton crew and a tight budget. I think Lakeside will keep going on it's own momentum into the foreseeable future unless something straight up collapses...