First, I don't know why, but I find this image amusing:
Attrition?(Also, if Dreadnoughts were always used, how come none of the Dreadnoughts from before the Heresy have survived? Bjorn the Fell-Handed is explicitly stated to be the oldest one of them all, and he didn't become a Dreadnought until long after the Heresy - one would think that if Dreadnoughts were actually commonly used even back then, at least some would have survived by sheer coincidence.)
Plus, what you state is not quite true. Bjorn is oldest pilot of dreadnought. Not machine. In fact, Codex: Blood Angels states that some of theirs fought on Terra during the Heresy, so it can't be right.
Why they listened to him? Well... He had that little thing called 'more marines than everyone else combined, twice'? He was the only person that could have (and kept) Imperium together post-Heresy? He was the only Primarch who actually knew how to rule a state? I suspect once immediate post-Heresy period ended whole Imperium come to accept him as the boss, or at least bigger part than everyone else.
Spiritual Liege would have thrown that right back and asked where Russ was during that battle, and pointed out that he kept 100.000 Heretical Marines at bay when the Wolf just kept running randomlyRuss, on the other hand, took a look around, noticed that the only way they were able to stop Horus in the first place was because of the massive strengths that the Legions could bring to bear against each other and told Guilliman to pull his head out of his arse, and then congratulated Guilliman for his integral part of the defense of Terra.
Um... Wolves were split, too.Russ...Was left alone. No-one messes with Russ.
Haven't latest IA introduced anti-Marine Chapters? Minotaurs? Executioners? Carcharodons?Sort of. Ultimately it's whether or not Terra approves of your Chapter, and, if they don't, what can they do about it, and whether or not it's worth the cost. In broad general terms, the only thing that can kill a Marine is another Marine. And most Chapters aren't willing to fight other Chapters for a variety of reasons.
The same Imperial Guard that held against 100+ Chapters worth of Chaos Marines on Armageddon, eliminating a few dozen worth permanently? Or the same Imperial Guard who failed to held CadiaSo Terra using Loyalist Marines to sanction 'Renegades' is usually out of the question unless it's for a really, really good reason (see Badab War), not just because 'they wont follow the Codex'. Which means they use other forces (Sisters, Guard, etc.) which aren't nearly as effective as actual Marines and generally fail at trying to wipe out 'Renegades'.
Why out of character?The main point of the Codex was to thin down individual political power, by thinning down the military might behind it. To Guilliman's credit (and wholly out of character), Guilliman actually did it to himself as well.