Chuck - John S. had nothing to do with the actual R&D phase of TMax, or the reason for it in the first place, but was brought in after the fact to help promote it for general photography. It was designed to replace several key prior films, including Super-XX, Plus X, Triassic X 320, and Tech Pan. It would be the new Jack of All Trades, and pretty darn competent at all those tasks, but only if one knew how to tweak it relative to each application. And old dogs don't like learning new tricks, or losing familiar films that respectively did just one trick very good, every single time, that they expected it to do, without changing exposure and development protocol.
Craig - the grain of Delta 100 is about the same as for TMY400, maybe a tiny bit less, but not as fine as TMX100, depending of course on the specific developer. ACROS, being orthopan rather than pan, is even a bit smaller, with excellent edge acutance. But again, it depends; Acros being a dual-emulsion film, if combined with the wrong development method, and you can get grain significantly larger than TMY400. I've done just-for-the-hell-of-it testing to prove that to myself, even though I had correctly developed Acros for many years.