I'm not sure how true this is.

If you consider low-stakes crimes, typically to get to a steady state of effectiveness you need at least some sort of bootstrapped period of ubiquitous enforcement. If that's impossible then I'm not sure you ever get to effectiveness.

If we're talking high-stakes, death-penalty-lottery-if-you-break-the-rules type stuff, then I think actually detection rate (i.e. consistent enforcement) is the biggest predictor of reduced rates, not severity of punishment.

That is incoherent. Laws that are not consistently enforced are by definition ineffective. For starters, you can at least grant that the law was ineffective for those who violated it and didn't get caught, an inevitable consequence of "intermittent" enforcement. More than that, inconsistent application of the law incentivizes more sophisticated ways to evade it, which means the people who do get caught are simply the ones with less money, resources, connections, etc. If your rejoinder here is that the law still functions as a deterrent to some degree, the onus is on you to prove that.

Let's also acknowledge that you're straying further and further from the central point of this particular discussion. This is not simply about "intermittent" enforcement. Enforcement of this rule will be extremely difficult, if not impossible, as the technology gets more sophisticated.

And laws can be completely useless when enforced intermittently.

Laws that are enforced and more importantly are enforceable have a much higher rate of making a difference. The same works here.