I kind of view it the opposite way. If the Bears are convinced that Clowney will be a star player, its less of a risk to trade up and take Clowney, because at least the Bears will know what they are getting (to some degree). Those other draft picks might not ever pan out, especially if the 1st round picks are in the mid/late round range. I think a top 5 pick is a safer bet than two 15 picks and a 115 pick. JMO
No draft pick is a sure thing, so I'd be more inclined to spend top dollar in free agency to get a proven performer than to get a draft choice cheaper for 4-5 years, but then have to pay more in free agency to fill the other spots in your roster.
IMO 2 15th picks are worth more than 1 top five pick becuase those 2 15th pick not only spreads the risk of failure over two players but it also has 2 rookie salaries which allow you to have more cap space to either extend/resign your own or be a player in free agency.
We clearly have different mindsets. Let's take Seattle for example. Their success was not because their top draft picks paid off, but more their non-first rounders hit. Thinking that before this season Chancellor, Sherman and Wilson were all making less than $1M/season. That allowed them to get guys like Avril, Harvin, Bennett and Chris Clemmons in free agency. Essentially having those draft picks outperform their draft spot allowed them to have additional funds to build a winner.
Looking at your logic I can't think of a time where someone who traded away quite a few picks to move up ended up having a champion contender in the years that follow. Correct me if I'm wrong, but it just hasn't been a strategy that has worked in the past.