I suspect they expected a lower Trump turnout.
My guess-and this is obviously only a guess, is that they expected Joe Biden to tank it with Democrats. He had low in-party support. Lower than Clinton. Low in-party support is always a strong indicator that the Democrats are about to bite it. Especially when Trump had strong internal-party support. Playing it safe though, they would assume that he'd be able to pull the numbers he got last time.
I think they expected Trump had lost some support, just much less than they had. So if the plan went perfectly, then the media could announce that Trump had lost support in key battleground states, a "clear repudiation of his presidency" and present Biden with respectful numbers that would have beaten Trump regardless. And if Trump pulled the same numbers, then they'd still be able to argue strong Biden support.
The cover of COVID was perfect for the Dems to explain the irregularities of the election. They also had support from the bench in certain states, allowing them to change the rules at the last minute. And with RGB on SCOTUS, they could trust her to try and sway the Bushite globalist who would probably look the other way in order to be rid of Trump.
The first complication in the plan probably came with the riots. The Democrats seemed to have expected that Trump would be the one to suffer, but it turned out that they suffered too--considerably more as Pelosi and others called for an end to the violence one or two months ago. That wasn't good, but it wasn't a problem. They could make up those votes given their advance timeframe. The second and more crippling issue was the loss of RGB and Trump putting a new SCOTUS on the bench. And they couldn't delay this judge with the same slimy tactics without both pissing her off and alienating key votes they needed (women, Hispanics, Catholics). The third complication was I think, the second debate. Biden had gone on record with comments that alienated PA voters. The fourth complication was election night itself.
As it turns out, Trump not only retained his previous support in 2016, but he had actually gained substantial support. Not only were the Democrats not going to get their repudiation narrative, but they were also having difficulty catching Trump, let alone exceeding his count to stand above 1%.