Are you ~seriously~ comparing replicating a Steinway Grand Piano with sampling to doing the same to a Mellotron?
I wouldn't.
Sampling a Steinway (or at least 'getting an accurate digital picture of one') is probably much easier as Steinways are way more consistent in their sound than a Mellotron. You can play the same note ten times on a Mellotron and they won't sound the same every time, in the same way my upright piano doesn't play the same way two days in a row. Steinways on the other hand have centuries of engineering refinement in them that gives you a very consistent sound.
I am also assured that how good a grand piano sounds is about 50% down to the piano and 50% down to the room.
Grand pianos will be around long after the last \u201ctape based\u201d Mellotron has rolled off the assembly line.
I don't think he has said otherwise. The longevity of the instrument isn't an issue anyone was discussing.