Love all the comments about Omegas lasting for ever.
Look at www.howmanyleft.co.uk and you'll see exactly how well Omegas last.
Comparing 2016 with the year 2000, something like 1% of Omegas on the road in 2000 are still running today. Yes: 1%
99% of Omegas have been scrapped. For good reason: people didn't want to buy used Omegas - not at 3 year, nor 5, nor 7 years old.
Look at the W210's from the same period and you'll see that typically a quarter of those cars are still on the road 16 years later- often with high mileages. People like them and keep them.
Have a look.
And you have solved your own puzzle.
It has nothing to do with how reliable Omegas are, but as you said that people didn't want to buy used Omegas. So many people snob Vauxhalls. It matters not that Omegas won't rust as much as W210s and cost less. It matters only the badge.
So are you rally surprised there are more W210s left? Are you really surprised that people would want to keep the plastic Mercedes star polished instead of the Vauxhall griffin? When you gloat "I drive a Mercedes" sounds way more impressive to the average person than if you say you drive a Vauxhall.