You keep conflating "physical age" with "economic life". (70 yr old home is already older than it's economic life).
That's an inconsistency in your reasoning. The economic life is how long on average a structure can be expected to contribute with little/no maintenance and updating. Not how long a property can be expected to contribute in any one specific location or when well maintained in order to slow it's decline. That's basic, and you shouldn't be just skipping over that distinction.
As for assumptions, there is no approach to value which isn't dependent on significant assumptions. So "assumptions" don't suddenly become a deal breaker just because it's the CA.
For your 70-yr old home, if the underlying site value is $100k and the value of the property in the market is $300k then what is the apparent contributory value of the improvements in their "as is" condition? And how would you explain that on a consistent basis when considering that in some locations that structure will contribute a lot more to the whole than in others? You can do that using the M&S methodology. You can't do that using most of the other methodologies I've seen appraisers use.
Nope - no conflating here, although I am impressed by your use of big words. Slightly out of context - confusing might have been a better term - but a valiant effort nonetheless.
(1) I had thought that the strong implication in my example would be that the 70 year old home would still be offering contribution under it's intended use - but that's my bad for not making that clear. So, back to my (now clear) example, how can a 70 year old home - that is still offering contribution under its intended use - have a TEL of 55 years?
In addition, while physical physical age is not the same as economic life, would you agree that there is a relationship between actual age and effective age? And, if so, you'd hopefully also agree that there is a relationship between effective age and TEL? In which case, logic would demand that there is also a relationship between actual age and economic life...
(2) unsupported assumptions are deal breakers - regardless of the approach.
(3) In your example, are you saying the underlying site value is $100k in every location? If so, and if the overall value is $300k in every location, then the contributory value of the improvements is $200k in every location... I'm guessing, though, that (a) the site value is not the same in every location, and (b) the overall value is not the same in every location. And there goes your consistency...