Uh... yeah, they can.
2019 was the worst year to do anything in support of Michael Jackson. His Spotify numbers are irrelevant; we all know he's still a massive pop culture figure and that people still listen to his music. But if Leaving Neverland could completely overshadowed commemorations of the tenth anniversary of his death, led to radio bans in several countries, and temporarily initiate the most intense "cancel MJ" movement we've yet seen (regardless of its success or lack thereof), do you really think a new album/single would've been a smart move? That would be like if Michael dropped an album in the middle of the 2005 trial -- no reasonable person would've said, "That was the best possible decision."
The Estate's radio silence from 2015 to 2018 was annoying. Their withdrawal from the public eye from 2019 to 2020 was completely justifiable, and (in my opinion) the right thing to do. The last two years gave people plenty of time to see the documentary for what it is and embrace Michael publicly once again. And ideally, 2021 is the perfect time for them to come back with a vengeance.