Dekeyser, Kronwall, Ericsson, Fischer. Off the top of my head those are the only dmen who have somewhat recently came up to Detroit while having waiver/stashing options available, or who have progressed into pretty large roles very quickly upon arrival.
The problem is that Detroit's had very few good young defensemen, especially compared to forward, where we have more cases of young players coming up and in quickly.
My general impression for players pushing their way onto the roster is that they just can't be as good or better than the bottom half of it. Whatever "upside" formula the team employs, they disregard it if they are figuring on the young player not being able to crack the top 3-4 on D or the top 6-8 at forward.
That theory holds up in analyzing pretty much all of Detroit's roster calls with young players over the past 15+ years, essentially Holland's tenure.
Do I need to point out that you're having to go back nearly 20 years to try to build a point around guys making the blueline? And even then Kronwall and Ericsson were 25 when they played their first full seasons in Detroit (or turned 25 that season, depending on when their respective birthdays fell), and Dekeyser came up when we were decimated by injuries and we were leaning on the likes of Brian Lashoff to get us through the season. Ficher got his start when roster limits were higher - something I wish they'd revisit considering the high energy games now being played from start to finish.
For the most part, I'd agree with you about Detroit's philosophy barring some outliers (Lashoff, Glendening, Andersson come immediately to mind), but that doesn't mean I think it's the best way of doing it. It's part of what led us to dumping ~$5m into Kindl/Smith, and another $4m into Quincey to fill out the bottom half of our blueline. I think you've talked about people looking at the mythical upsides of 25 year olds when looking at the roster, but that's really what the wings did with Smith and Kindl, and both have turned out to be debatable moves at best.
Has Detroit had a bunch of top3 quality D knocking on the door? No, at this point it's probably safe to say they haven't, but they've had several who could have taken those 4-7 jobs at a fraction of the cost and we wouldn't have lost anything on the ice. And we've sunk a lot of money into those spots to fill them poorly.
Going back to Fischer, though, and roster limits. I think it's safe to say the regular season has become more competitive with cap ceiling/floor, and the games have become more stressful from the beginning of the season on. We're also seeing a growing body of work about the negative effects of travel/playing/lack of sleep/etc. considering costs are for the most part controlled, I don't see why it's not being looked at to expand the rosters again. It would give teams more flexibility with subbing players in/out to keep them fresh, and allow slower grooming of players - like the Wings did with Fischer in his rookie year - since they don't have to play him every night. To me it's win/win for the league and the players (more jobs).