What does everyone think about the turbo diesel engines? Is there a future for the turbo diesels on the Comanche airframes? Or will the upgrade exceed the value and longevity of the entire airplane?

Everything I've read about the turbo-diesels indicates the cost for engine replacement would far exceed the airframe value. Hope this changes in time.

My perspective is European so may differ from the US view. I put the first Thierlert diesel in a C172 in the UK about a decade ago and shortly after that rebuilt a C182 with an SMA diesel. I have quite a bit of experience including lots of early adopter troubles. In spite of this I still think diesels have advantages - mainly jet A1, greater reliability than AVGAS engines (now they have matured) and better fuel specifics.
Although a PA 30 with turbo diesels of somewhat more than 120hp would have a lot going for it I think that the cost of certification would make little sense given the relatively small number of airframes and the fact that most owners value the PA30 as a low cost twin. One indicator of change would be to see how the market starts to value aircraft with really major upgrades. They tend to be referenced back to quite basic airframes and are seen as unrealistically expensive. It might be fairer to reference them to more recent models with similar capabilities in which case they are cheap. My PA 30 has a very large number of STCs, several of them quite major.and loads of avionics. It would need to sell for three times the price of an airframe with minimal upgrading to come anywhere near recouping the investment. I expect to take a major hit when I stop flying and sell but in the meantime it does what I want. I take some consolation in believing that this 50 year old aircraft should still be flying effectively for several decades to come.

I don't think most owners would ever get a return on the cost of changing over.

The only reason I'd consider a diesel would be if Avgas or any suitable alternative wasn't available.

Interestingly Cessna dumped the diesel engine for the C182, and that was after they'd built a good number of C182's ready for sale with the diesel engine. They pulled the engines out and re-engined the airframes with Avgas engines. They've have never said why to my knowledge.

As for diesels coming of age, that "Free Beer Tomorrow" sign has been hanging there for an awfully long time now.