First, on the legal side: the firm can leave the position open, or hire a full or temporary person for it, but when the parent returns they get their old job back (or an equivalent one). Also it's not exactly 52 weeks parental leave, it's 13 weeks medical leave (for the mother expressly) and 39 weeks for either parent, to be split as desired. And that's just the paid component (which is paid by the unemployment insurance system). You're also able to take unpaid leave as the other parent. Same difference to the employer, of course. They're not on the hook for the direct cost.
What generally happens is the firm hires a person on a one year term, and then at the end of the year the original person comes back and the replacement person is either kept on (if they were good) or not (if they weren't). It's a decent way to hire new people, actually.
>>What if the person takes parental leave again after 1 month back? Do you accept that you've torpedoed some role? Do you try to slowly phase out this person's job description and offload their work onto their coworkers?
In Canada you can't take parental leave simply at any time, it's tied to the birth of the kid (wth leeway). It's very unusual to have a child 13 months after the previous one, and there are rules about how often you can take the paid unemployment insurance (tied to how much you've worked over the last year). But yes, you'd be entitled to leave from you job.
>>More succinctly, what if someone is definitely necessary in the near-day-to-day, and does not do work that is interchangeable with other employees?
A well-structured organization typically doesn't have very many of these people, because it's a ticking time bomb. That person will leave; they'll quit or get fired or get hit by a bus or get cancer or take a three week vacation and the business has to figure it out. And babies don't really sneak up on you that much - there's plenty of time to train someone, etc.
>>I suspect that firms have learned to deal with it, but by doing things like passively selecting against women and younger people for critical roles. Is that how they manage?
I've never seen any data that Canada has poorer gender equality or more age discrimination than the US.
Overall I think that the parental leave laws in Canada do make things marginally more difficult for businesses but significantly easier for new parents. That's a tradeoff I consider reasonable.