I have done it, quite a few times. It has happened, once in a while, that someone resigns abruptly, which can leave the group in a bit of a pickle, although sometimes personal circumstances make this understandable, sometimes less so. There was one time, someone on a committee the Sage was on did almost everything. Treasurer, secretary, most of the work, although no one had expected her to, she was determined. The Sage was chairman. I warned him, don't rely on her. It's too much, it's "too hot not to cool down." No one can keep that going for long, and because of the person she is, she won't ask for help or acknowledge any problem, she'll just quit. And so she did.
I always start planning my exit strategy in good time before I resign. It usually takes at least two years. My aim is not to be missed at all, not to leave anyone in the lurch (I still feel the responsibility to the organisation, after all) and to help a successor to feel confident. Normally, I make a list of weekly, monthly and occasional events and how far in advance to plan them. No one has ever done as much for me, I jolly well wish they had. I involve the person who is to take over, so that it's not all new for them - I identify my successor as early as possible, of course, though not in secret. If it's all discussed reasonably, then people accept the decision. Even if they're horrified at the start, the idea soon gets absorbed into the brain.
It's different, being a volunteer in a position of responsibility to having a similar paid job. There, the objective is usually to be indispensable, to keep your place on the slippery pole, and if you resign then the odds are that you will have nothing to do with your successor. The difficult part about being a volunteer is that you have more of a moral responsibility - if you aren't enjoying your job, you aren't earning enough, you want more or less to do, you look for a new one and hand in your notice. However inconvenient for your boss, you've every right. But when things in a society, school or voluntary organisation are going pear-shaped, you are obliged to stay and help put it right.
The other time you can't leave is when there are so few helpers that it really would be a problem for the ones that remain. Or if you have a skill that no one else there has. That's why a treasurer can get landed with the job for a long time. Or an organist.
PS - No sign of the baby yet. He's still a very large bump. Can't be long now though, surely!