With the price of diesel the way it is now, there is no way in hell I would consider a diesel swap into a gasser. I don't feel the hassle of a swap is worth it either. OTH, if I had an unlimited supply of good WVO, I would just run the 300D I already have. It would make a good daily driver.
I would go the other way around and drop a gasser or a SBC into a diesel. The thing is I already have a gasser 123 and it suits me fine as is. It's been cheaper to run it than a diesel model for quite some time now.

You're absolutely right, I have watched in horror as diesel prices have climbed past gas prices.
But it all depends on what you use your car for.

If you're using your car for daily commuting, go buy the newest, most fuel efficient vehicle you can buy, that runs on whatever is cheapest right now. Don't even bother with a 1970-something Mercedes. Same goes with environmental arguments.

If you're using your car as a hobby vehicle, enjoying a few hundred miles per year to meetings, Sunday driving or a holiday trip, the price of fuel hardly matters, if diesel is 10% cheaper or more expensive than gasoline.

If you want to preserve your car in factory delivered mint condition (or pick another point of time from the vehicles life) until armageddon day, gas/diesel conversions are out of question and price of fuel doesn't matter.

My personal reasons are, in no particular order: I like diesel fuel better than gasoline, the smell of it. I like diesel engines better than gassers, they feel more primitive, simpler, more robust and logical. I like the fact that my diesel engine can use vegetable oil (even if some members disagree), and this can help me economically in times when diesel fuel is expensive (assuming I do this the proper way and take appropriate precautions). And, last but not least, I like hanging on the shop floor, lifting in and out engines, manufacturing pieces of oil filter relocation kits, restoring interiors, and you name it. This is a hobby for me and when diesel prices go past gas, I don't like it, but it doesn't kill me either.

Whatever makes you happy, try to keep doing it.. :o

ps. If I were to build the ultimate vehicle for me, cost being no issue, I would start with a W108 and either go with a state of the art 2008 CDI Bluetek diesel engine + 6 speed dual clutcher in it, or build a series hybrid with a very small diesel engine and a larger battery pack + electric motor.. But this all costs money and that's what I'm trying to avoid right now with my OM617-projects.

I have a feeling the price of gas will pass diesel again soon. After all diesel is less expensive to make.

To me a W108 with a 617 turbo and a manual would be one serious brass ring.:D

I agree with most of the points Tomas made. 98% of people do buy modern cars, that is why you see so few "oldtimers".

However, personally I dislike modern stuff - boring, plastic, built to a price and cacooned in electronics and insulation. I've tried them and always gone back to classics, even for daily use.
Every trip in an old car has a sense of occasion and fun for me. Most people don't understand this, but I actually like being able to fix and maintain my cars (and actually knowing that is simple and mechanical and I will be able to fix it)!
I could afford buy a new car (not an S class admittedly!) and take it to the dealers but I choose not to.

As to gas vs diesel. Well, there is a very slight premium to pay for diesel, but the increased economy more than makes up for it. That is why diesel cars are now in the majority in the UK.
In the USA the situation is totally different of course, if I lived there I'd run a 6.3 as a daily driver!