I think whenever this question comes up it's worth addressing just how brilliant this Hawthorn, and to an extent, Sydney sides have been.
I think it's come up quite a few times, but Bucks AND Derek Hine established after our 2012 Prelim loss that we simply didn't have the talent to keep up with the rate they were progressing, and it's hard to find anyone that doesn't agree, this Hawthorn side is simply one of the best of the past few decades, and even if all the blokes you mentioned had stayed, it'd still be hard to see us beating Hawthorn in a granny, we barely scraped past them in the Prelim.
And who's to say Malthouse wouldn't have pulled a John Worsfold, who despite being a great coach, simply lost his passion for coaching, and thus, his ability. It's easy to say Malthouse could have coached to a high standard until he dies, but there's a reason that every great coach has an expiry date, Malthouse has been coaching for such a long time, and it's an absolutely fantastic achievement which we will celebrate by smashing Carlton in round 5 this year, but every coach has an end, and baring the succession plan, it could very well have happened the last few years had he had stayed on.
Obviously we will never know.
Anyone who follows the EPL would have seen the transition that Manchester United went through once Ferguson left, he had the liberty to chose a successor and even still Manchester United fumbled last season and sacked that very successor, there was never going to be smooth transition to a new coach, succession plan or not and that's probably a point worth emphasizing.
I absolutely agree with every point you've made OP, and like you, rather then dwelling on how we could have potentially been a dynasty (we wouldn't have) i'm excited with the opportunity our youngsters have been presented this year to prove the doubters wrong.
At the members meeting at the end of last season, Bucks and Co emphasized we've been rebuilding for 2016 onwards, when we'll be up against the likes of the Bulldogs, GC, GWS and obviously Port at the top, nothing I have seen has changed that vision, and i absolutely agree we'll be a contender in 2016 onwards if this team stays healthy.