The batteries lose very little capacity as long as they are thermally managed and never fully charged or discharged. GM's Gen-1 Volt has a "charge window" of 20% to 85%. They increased a bit with the Gen2 Volt after getting real-world data from Gen-1 Volts in the wild.
There are 2011/2012 Chevy Volts in the wild with around 100K EV miles and no apparent battery degradation.
Here is all-time the leader, "Sparkie",a GM employee from Michigan:
And mine... ;) (Yes, I'm totally biased)
You can browse the leaderboard, sort by "EV miles" and browse others.
As for Tesla, I don't think their batteries are quite up to the level of GM's, but they do employ active battery cooling and a charge window.
The one model that doesn't have active liquid cooling, the Nissan Leaf, is notorious for suffering significant range degradation after only a few years.