Why is it likely to be a hilarious underestimate? That seems like a statement that needs some support. Graphs don't go up forever; growth curves level off; growth gets harder the bigger you get. Linear growth forever should not be the default assumption for any energy technology, even one that is growing fast now.

Or do I misunderstand what you're saying?

But why does the curve level off? Supply and demand. Assuming that renewable energy is superior to fossil fuel from both environmental and cost perspective (it is, or soon will be), then it can reasonably be expected to grow until all fossil-based sources have been displaced, and it is no longer profitable to build more capacity.

Because people want cheap, cleaner electricity and we haven't used up all that much space yet. Manufacturing capacity is still growing, the rest of the industry is still growing, it's not likely that there will be a year in the next 20 where global installations are smaller than this year.