because I can't see the reason why would they even apply for a OWP if they won't change employers anyway
Many reasons. Having an OWP makes things faster in moving inside the company. Or someone is or about to be laid off or fired and they want to ensure that they have a job before that eventuality hits them. Or they got promoted and their job title, NOC or duties changed. Or they want to live else where in canada and are on a tight deadline.
Many not so great employers do not give you the hike because they believe till you do not get a PR you have no recourse. Having an OWP acts as a passive agressive move. You update your HR that now you have an OWP, they will read it as a precursor of you leaving the company. At the very least, it chills the managers in small companies, makes them less aggressive. The moment a shitty person knows that other party has a recourse their tone and tenor changes.
Technically speaking it does, if someone in their closed work permit P1, gets an open work permit P2, and changes employer afterwards (because I can't see the reason why would they even apply for a OWP if they won't change employers anyway), the P1 work permit gets invalidated at that point because it no longer fulfills the original purpose, and you can't simply fall back to that in the event the P2 gets invalidated because your student spouse graduated and decided not to apply for, or is ineligible for PGWP. If you want to stay, then you need to move to another status such as another closed work permit.
Actually P1 never gets invalidated. Till it expires. This is one of the curious property of WPs. Canada government does not have any means to invalidate a specific WP that has validity left. They can only deport you and then it and other statuses ceases. That or they can disallow you to enter Canada. This is why when people are laid off while being on closed work permit, they are often told to NOT leave the country. Because even though they have authorization to remain in Canada, they can be denied re-entry if CBSA finds that you do not have a job. When one loses their job, they are often told to NOT attempt a flagpole because it can happen that they are denied and also denied from re-entering canada.
How do I know? I worked in two different offices of my company in two different cities. I once held closed work permit for Vancouver and later a permit for Victoria. Both were valid and allowed me to work in both cities. I had asked my lawyer specifically about it.
What else it means? It means, say you quit your job while on a closed work permit (which had substential validity left), went to work for another employer on an open work permit with substentially lower validity and then switched back to your previous employer (may be they decided to give you a large hike after all), you can work till your closed work permit's valdity runs out (Assuming P1 had more validity). Till either P1 and P2 have validity, you can stay in canada.
But I digress, the original argument I was defending was that, these student applicants should have done the due diligence of what are they getting themselves into. If they entered Canada, was issued a student permit with a leave by XYZ date, and then did NOT APPLY for any other status, nor seek any legal options to extend their stay or simply put there aren't any eligible options available to them when they are looking for it, then they should leave Canada by XYZ no questions asked, and they shouldn't expect an auto-extend study permit, or auto-transition to work permit. The burden of understanding Canada's law falls upon the immigrant or a lawyer working for them, and not by the Canadian government.
My original point was, Canadian government misrepresents facts on its official permits. And that is not some simple incidental fact, thats a condition on the permit. Its legally binding. You can not research away a lie. When Canadian government is lying and misrepresenting, it is not fair to expect students to always be on the top of their legal game.
While you make a point that the Canada's wording on VISAs and permits could be improved (I disagree on that, but let's agree to disagree at this point), these students, should they had an intent to stay longer in Canada, should have researched on what are the available opportunities and think their journey through before it even started; instead of moping around and blaming the Canadian government of letting so many of them in (particularly so many of them in the same degree).
When it comes to question of legal documents, words are not improved. They are correct or there are consequences. If I hypothetically misrepresent my marriage on my WP (say, for example), there are consequences. This is why I find it very distasteful for Canadian government to misrepresent reality. Which is why I am not really keen on holding students to be really wrong in messing up their immigration.
But the courts can't change the current policy of VISA-hopping they can only decide if it's legal or not, it can't change the number of people that the Federal Government decides to let in, it can't mandate public universities to lower tuition fees for international students for equality. The courts can't even pick who gets a chance to get in like which skillsets should be prioritized, or which degrees should get more student VISAs, or which companies should get an approved LMIAs. That duty befalls on the policy-makers in Ottawa, and the provinces, and for that to change, you need the government to change, and only voting can change that - something that temporary residents can't do.
Actually courts can hold a policy to be in contempt of "Canadian Values" or "Charter" or "principles of natural justice". It has happened in past. And when that happens, government is forced to change or amend the policy. The JR process is very expensive however. At times, Canadian courts specifically answer questions about laws by a process called certified questions. Some of these are of general importance and do over turn regulations policies etc or provide relief to the other party.