I accidentally left the game running, winter happened and all my kittens starved.
It’s sad but it took almost no time before new kittens replaced them. Is there a major downside to starving all the kittens in the winter and replacing them in the spring?
No, it’s not really a problem. Assuming you’re there to re-assign them.
The big problem is if you make sure you can survive a normal winter (-75% catnip rate), leave the game idle for a while (say overnight), and at some point, a cold winter hits (-90% catnip rate). If a kitten dies while you’re idling, then the new kitten to arrive won’t be assigned a job until you get back.
There are a few achievements related to starving kittens, though. You need to starve kittens for two of them (one at ten dead kittens, one at 100), and another comes for reaching 50 kittens without any dying. You don’t need to worry too much about missing that last one, though; it’s easier after a reset or two anyway.
Of course, the real question is: are you going to live with yourself after letting a poor innocent kitten starve to death?