Based on prospective studies of post-infection illness, there is a declining curve of people still sick over the two years following onset. Typically, from memory, it's around 10% of people infected with certain trigger diseases who still have symptoms that significant affect daily life at 6 months, declining down to 2% and then even less. So, most people with persisting symptoms at 3 or 4 months do recover; recovery rates really are high. Think of people who spend a few or 6 months recovering from EBV - there are a lot of them.
Just lately we've seen a couple of papers I think, that suggest lower recovery rates in people who have post-Covid ME/CFS at 6 months. But, these are fairly small studies with possible biases. We could definitely use more good information about illness trajectories. I would have thought we'd have lots of robust information on that by now. But we don't seem to.