It wouldn't have made sense to do a monolithic model if they were intending to release single chiplet models. The RDNA2 refresh will be just fine below Navi 33.
We really don't know to what degree their MCM approach has performance issues that a monolithic chip won't have. The first Zen CPUs had some behavior quirks that AMD was only able to iron out as they gained experience and iterated on the design. Maybe some of that helps with potential problems with GPUs, but the fact that they haven't done it until now suggests there are some hurdles.
But putting that aside, there will always be IO dies that can't connect to a full number of chiplets due to some defect. There's no reason for AMD to waste silicon if it can be turned into a product of some sort. Using chiplets makes it easier to create a wide variety of products with a limited number of dies. It's the same idea with Zen where you can get 1 or 2 chiplet Ryzen desktop parts or get a Threadripper CPU built from the same chiplets if you need more power.
The whole point of using chiplets is so you can use the same silicon to make a wide variety of products and be flexible enough to adjust to market conditions. Making multiple monolithic dies to cover those products is less cost effective and isn't nearly as flexible.
The fact that we're seeing 3 different Navi dies, one of which is monolithic would seem to suggest to me that there are still some kinks to be worked out when it comes to designing MCM GPUs. Otherwise AMD would have just created a single Navi chiplet that could be used in any number necessary to hit a particular performance level.