Dan, why do you think the 6.3 is declining in popularity (in the US)?
I have had this chat with the locals. I imagine cars have their first peak in 'value' when those who grew up admiring them can finally afford them. But then after years, like now, the 6.3 becomes a bit scary as who can repair them?! finally, a second peak comes when collectors realise numbers of such cars become quite scarce. Here in Australia, 6.3s are pretty scarce, and I think they have reached a climbing phase. Personally (and I think ive got good contacts to fix them as I cant!), id be too scared to own one. The 6.9 however, seems a more manageable automobile for an aspiring classic car owner.
Again, as for the values, I think the MBs are just catching up to domestic products. Each country has its own home grown bias for cars. Here in Australia, Mercedes fans often shake their heads when they see similar aged Holdens or Fords which aren't anywhere near the same quality as our cars, sell for obscene values. In the US you have your various brands and muscle cars, and in the UK, you have Rollers, Jensens, Jags and so on. Now, as numbers thin out, our cars are finally picking up.
I dont see much of a problem with this car, its very clean and straight. Despite attempts, you will not be able to put together as nice a car as this with the resto of a cheapo 116 without a massive investment (of time and money).
Nathan yes I think you nailed it in that my theory is that the baby boomers are (literally) dropping out of the market. Take a look at how auction houses like Barret Jackson have seen a drop off in values on cars from the 50s 60s and to a certain extent 70s too. Simply this was the sweet spot for boomers with disposable income finally being able to buy the cars of their childhood dreams.
Forward now to the Gen X crowd and lesser extent Millennials and now we are seeing cars from the 80s rocketing in value. Look at 126 values for example and E30 BMW.
Cars from the 70s are in a bit of a "no man's land" it seems value wise.
As for the 6.9 indeed it's the only M100 you could realistically live with on a semi daily basis. 6.3 parts are priced in the stratosphere and 600 parts are in another galaxy!
Also vintage Benzes are very much an acquired taste for the average North American. Land yachts are still very much the car of choice for middle America trying to "make America great again"...
So we have a perfect storm for very limited demand for M100 cars in NA. Again I think the 6.9 is now finally not the ugly girl at the M100 dance anymore.
This is of course just my theory and not to be taken as the gospel

believe me I really want to see 6.9 values come into their own and I think the tide is finally turning
