The Yes campaign was by any standards an extraordinary phenomenon - and yet its only concrete achievement was the Smith package. (I know some people would say the SNP landslide was also a direct result of the Yes campaign, and that's true, but election results are ephemeral.) If the new powers had never materialised, there was a danger that we could have ended up looking back at the 2014 referendum as an event that ultimately achieved absolutely nothing - which is pretty much how the sovereignty movement in Quebec look back at their near miss in 1995.
It may well be that the UK government honestly thinks that more devolution is in itself a trap for the SNP - but they're wrong, just as every previous UK government transferring powers as a "brilliant tactic" has been proved wrong. Consider this, for example - how much harder is it going to be for the BBC to resist the case for a Scottish Six now that income tax rates will be entirely set in Scotland? How can a one-size-fits-all "national" news programme broadcast from London possibly cope with that degree of asymmetric devolution?