(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.)
There are 2 possible solutions for this, firstly that the one Dreadnought who managet to live though all of this is in fact Bjorn, the rest of them has managet to blow themself up though the following 10.000 years of warfare.

The second solution is that Bjorn is the only Dreadnough who is being kept in a stasis crypt to prolong his lifespan, and that all the other dreadnoughs from that time has lost the battle against entropy.