Right now, all win rates indicate really is how quick you are to switch off a hero when they're not working and how good you are at picking a different hero that does work in that current situation.

To add on here, this is only one potential aspect, and part of a larger overall problem the community has with evaluating statistics (see also: why private profiles are actually a very good thing).

Let's put aside the ideology with switching and all of the associated shenanigans one can do to pump the numbers. Instead, let's look at a situation that can occur naturally due to the variability in game length:

Let's say a player picks a given hero and plays four games. In the first three games, their team absolutely shits on the opposing team in record time: 100-0, 2-0 on three capture maps, netting around 15 minutes of playtime for those maps. In the fourth map, they get an eight-round endurance run on some 2cp map and lose, with 16 minutes of playtime on that one map. Their overall winrate will be under 50%, even though they're 3-1 on the session.

And these are now just two of many situations that can occur which make stats like winrate not necessarily indicative of anything on its own. And since we don't have any public-facing way of determining what maps the wins/losses come in (and in what percentages on those maps), it's almost useless in most cases.