It was clearly a very popular machine with users; there was no real business case for producing clones, and yet there were no less than 3 companies that attempted PDP-10 clones -- one of them in the mid 1990's! -- in addition to the one-off clone at Xerox PARC. Ritche and Thompson wanted one at Bell Labs to develop early Unix (and settled for a PDP-11 instead). And DEC threw it completely away in 1983 by stopping development on the successor machines. They could have done one at least more generation, and it would have been at least a modest success and kept some of those users on board the DEC train.
The PARC clones were two-off.
Even as a PDP-10 fan, I have to admit that the PDP-10 was not exactly the wave of the future during the 1970s. It had a decent niche and a steadfast following, but sooner or later it would have disappeared in favor of 8-bit byte addressed computers.