You'll also be surprised at how well people step up. You're going to be doing two productive things here: Bonding and giving room for growth to others.
As someone who did have to "step up", I think I did a fine job, but I wasn't exactly thrilled about needing to do 2 jobs now: my old job and the job of the person out on leave. For no additional pay.
I absolutely believe parents should take leave, and while the exact amount may be up for debate, I think that, generally, paid leave in the US is woefully short. But I think it's really shitty to just pretend the extra burden on your colleagues doesn't exist when someone takes leave, or that they'll necessarily see it as a great "opportunity".
It seems like either they really assigned all of the leave-taker's job to you, and that seems excessive, or this person was doing a TON of stuff. Were there no other people who could take over parts of the leave-taker's work?
I’m sure the impact to the parent commentor could have been mitigated, but absolutely sure that it almost always isn’t.
In this case, the startup was young, and (as is usually the case for startups) we had too much to do as it was. Also, coincidentally, we had a bunch of people having babies around the same time. There were simply not that many other people who could take on the work.
I think that's primarily the point I want to make: in a sizable corporation, probably a quarter of the company could disappear for a year or two and it wouldn't make any difference. A young startup just doesn't have that amount of spare capacity, so it's not realistic to think that going on extended leave (for whatever reason) won't have a large impact on the people that remain.
PS - if you want to argue that by "one quarter" should really be a third or 20% or 15% or whatever, knock yourself out. Point being that large corporations have a lot more redundancy that small startups, so the impact of someone leaving for an extended period is vastly different.