The S&W TDA line, descended from the first Model 39, has a lot of handgun history wrapped up in it. For instance, the S&W 645 was the first successful double-action auto in .45 ACP. Its successor the 4506 went on to sell well and helped usher in many double-action .45 ACP pistols. The 1006 was the first successful TDA in 10mm Auto. Had it not been for the 1076 debacle there might have been a future for this configuration in 10mm Auto. The 4006 was S&W's first pistol chambered for its own .40 S&W cartridge and was popular amongst law enforcement for a short time. Of course, it was displaced in this group by Glock and Sig.
The 5906 in 9x19 remains a cult favorite. Were it made still in a decocker only version without the dreaded magazine disconnect device, it would sell. I would buy one. Remember the Gun-of-the week journey S&W took around 1990 with its third-generation pistols? It drove collectors crazy, but for a few months there were DAO & Sig-style decocker pistols produced that proved how versatile the line up of third-generation pistols was. Finally, let us not forget the 3913 and variants that demonstrated a good concealed-carry weapon could be made with just the right balance and heft. It is all gone now. The new M&P pistols may make more sense in today's market, but they lack the mystique of metal on metal.