Remember Broadwell? Most people probably don't. If you blinked, you missed it. The only 2 desktop variants Intel released were on June 2nd, 2015. Skylake was released only 2 1/2 months later on August 15th, 2015. So there is precedent for Intel releasing CPU's and then going, "Just kidding. Here are the real CPU's we're releasing." If Rocket Lake is ready, I can't see Intel holding it back until next year, especially if the Ryzen 4000 series put a beating on Comet Lake.
I actually have a laptop with a Broadwell CPU, though they offered a full lineup in the mobile space, and those processors were "current-gen" for longer.
However, as far as the desktop CPUs are concerned, there were a lot of differences compared to what we are seeing with the launch of Comet Lake. As you said, Broadwell only offered two mainstream models on the desktop, one i7 and one i5, whereas Comet Lake is offering a full lineup containing 32 different processor variations, including i9s, i7s, i5s, i3s, Pentiums and Celerons. That doesn't exactly sound like something they plan to replace in a matter of months.
Also, Broadwell desktop CPUs were designed to plug into existing motherboards, so they were still arguably a viable option even after Skylake launched. They brought much improved integrated graphics to the platform, along with a bit better efficiency on a new process node, but not much else. Comet Lake, on the other hand, needs new motherboards, and is increasing thread counts at any given price point. Even if efficiency hasn't really improved, adding SMT puts their core and thread counts largely on par with AMD's current offerings, and their higher clock rates should allow most Comet Lake processors to match or take the lead from AMD in terms of multithreaded performance. That seems like a product that should be able to stand on its own, and Intel has generally taken their time in responding to new AMD product launches.
An Intel roadmap leaked last year indicated that Comet Lake would be launching in Q2 of 2020, and Rocket Lake in Q2 2021, and given the accurate timing of Comet Lake's launch, I haven't seen anything suggesting that roadmap has been changed substantially. People expecting Rocket Lake this year are probably going to be disappointed. And with what little we know about Rocket Lake so far, its very possible that it might not even be much of an improvement over Comet Lake.