I would like to point out that there is indeed a 10,000+ year old Dreadnought stomping around the 41st Millenium.
Bjorn the Fellhanded was part of Primarch Leman Russ's Wolf Guard during the Horus Heresy in the 31st Millenium. Russ went on a quest, leaving Bjorn behind to lead the Space Wolves as the first Great Wolf. He fell in battle and as most Space Marines too valuable to lose, was interred in the Battle-Sarcophogi as you mention in your post.
Bjorn is STILL alive and well and battling in the 41st Millenium. In times after the Horus Heresy there were times that Bjorn slept in the Fang and was only woken up during the Feast of Russ to tell the tales of the Primarch.
When needed, however, he is roused to war!
Other extremely old Dreadnoughts include Bray'arth Ashmantle of the Salamanders and some others...
Oh shit! I knew Bjorn was widely considered to be the oldest dreadnaught (and pretty nuts to boot) but I had no idea he was from way back when! Thanks for this correction, deff falling down this lore-hole for a few hours later :P
Hehehe, it is an awesome universe.
Well, Bjorn is not stupid. He can lead an army and has the Ancient Tactician rule, because he is a sharp commander.