Because the Omega was well over double the price new... comparing a 21-28 mpg exec saloon with a diseasal grotbox isn't in any way, shape or form a fair fight.
But that wasn't the point of the OP, or my post. By your reckoning the "grotbox" the op suggested wouldn't be comparable either. Sure its petrol but that doesn't even have climate or heated seats, or any of the other items you listed.
Besides, anyone of sound mind would buy a delivery mileage pre reg Astra for £13k
Really?
Anyone would pay the same money for a less economical car, with less equipment, and with an extra owner on the V5, oh and with 4 years less warranty. Interesting definition of sound mind.
neither of those suggestions have heated rear seats, a sunroof, dual zone climate control or 17" wheels as standard. And comparing like for like physically,
Nor does the OP's suggested car, hence my point. Those characteristics are clearly not relevant to the OP's replacement choice. Nowhere in the original post are level of features mentioned.
using the Omega as a starting point is very relevant to this debate because most of those involved are thinking towards Omega replacement
So let me get this straight, you're saying if I buy a New big saloon car, it'll cost me more than on Old big saloon car......

. Well Shut The Front Door! I never would have guessed that!

What my post was trying to demonstrate that the superficial calculation of "Finance payments on new small car vs cost to maintain my old one" (which was the OP's starting point) needs a greater level of analysis before it is a useful starting point for discussion.