I hear what you're saying.
But the argument for your question is that with Miller on the team he's taking upwards of 20+min/night, all first rep PP, all important match-ups,, tons of important draws, all prime offensive zone starts (and even the important d-zone ones I've noticed lately) - all of these opportunities are not given to younger players (or at the very least, less of these opportunities are given to younger guys). This isn't an argument for/against trading Miller, just a reply to your question.
I know it's apples to oranges, but at my last workplace my manager told me to find another job at one point - I was shocked - but he actually had my best interests in mind - he said I would never advance in the company with the senior managers I had above me as they would take all prime opportunities and likely be with the company for some time. That's exactly what happened - eventually I moved on and surpassed my previous management position by leaps and bounds with my new company. The poor colleague I had who was doing my equivalent job (different department) 4 years ago at the old company is doing the exact same job with the exact same pay and the exact same benefits.
It's not that young guys 'can't' take steps forward with Miller, but nobody can argue he's not going to get all the prime opportunities in all situations before BB goes to a 2nd or 3rd line guy.
That said, I've always sided with the managers/coaches in the league who say that the NHL is not a developmental league, so in the big picture, if Miller stays and keeps playing at his present level or close to it, of course he should be getting all the prime opportunities - it's clear he's taking advantage of them.