Um, 50-75 ships of the line would be cruisers and up. Escorts etc would probably be closer to 250 ships. If I'm reading fluff right, Segmentum Obscuris is unusual in retaining it's older crusiers for service. From the references to ship wreckers, ships get 'recycled' on a fairly regular basis as they end thier operational lives (on average 1500-2000 years for cruisers and escorts. Longer for Battleships and grand cruisers. However, these numbers are probably skewed due to losses)

It's a difference of style.

The ork does not live in a ten by ten room with a treasure chest. He travels in warbands of fifty other orks, and cleaves his foes with a giant steel cleaver. The elf does not live in some tree and grow mint. He jumps from world to world frying people with his mind and cutting through tanks like they were butter. The paladin does not ride from kingdom to kingdom in his platemail. He rides from world to world in a millennia old battle cathedral wearing terminator armor. The rogue does not slink through shadows in some cave or buckle swashes in his search for riches to plunder. He sails the deep black of space with his crew of knaves in search of riches, fame, and power. War brings profit and renown to the cunning and the strong.

And in the darkness of the far future, there is only War.