I think they are delaying it almost exclusively for attendance. They already have Ratatouille (and maybe Harmonius) as the "new thing" for this year and have a limit on how many people they can even accommodate until they get closer to "normal". Heck, they even have the 50th as a draw for whatever stuff then do. They don't want additional draws until they can really capitalize on it. Hence, 2022 opening.
The fact that they'll save some money in the short term and defer that spending is a nice side benefit. But given the amount of work already done, I don't think we are talking about an overwhelming amount of spending to save in the grand scheme of things for Disney.