Equally many people stay for 3 years and apply for citizenship as they are leaving.
With the exception of spouses, I remain strongly skeptical that anyone is able to do actually do this. Unlike you, I've gone through this myself, and while on paper it's theoretically possible, in practice there are a lot roadblocks and obstacles. (It's not that you wouldn't get there eventually, but you'd get delayed beyond those three years. Now I got it especially rough due to how my personal timing stacked against COVID, but even without that I'm quite confident in my estimate that I'd have been looking at 6+ years.)
You can literally count time as a visitor towards citizenship which is insane.
But you also can only carry a maximum of one year's reduction on the waiting time. (A personal sore point for me since I have so much time on a work permit.) Aside from spouses (and common-law folks) visiting their native-born Canadian citizen better halves (prior to being able to file the family class app), I'm highly skeptical that anyone has been able to take advantage of this.
For spouses specifically, I'm not caring much either way. Basically, a spouse who does this successfully has two years of living with the sponsor on a visitor record plus two years as a PR in Canada (for four total years), versus either waiting outland and then living three years with PR in Canada or five total years in Canada (the two years of living in Canada on the visitor record plus the three years of living as a PR in Canada).
In other cases many are supported for many years and when they finally are paying more into the system than benefiting from it they leave.
The only class I feel it is fair to leverage this against is the economic class (who are invited for their ability to benefit Canada). It's obviously a clear humanitarian violation to require refugees to prove they'll benefit the Canadian economy from day one, and requiring this of spouses doesn't sit right somehow.
(For spouses, other countries like Australia and the UK are requiring stronger proofs that the sponsor is, well, rich, I guess with the idea being that having a rich spouse prevents the other spouse from needing to go on support - and in the event of a breakup they'd prefer to find a new wealthy spouse rather than going on government support.)
en other countries have increased time until citizenship.
I'm actually not aware of this. I was aware that Germany recently allowed dual citizenship and shrank the time to three years - matching Canada's theoretical (though I would not be surprised if, like Canada itself, the red tape in Germany means that it effective takes at least a couple of years longer). Though this might also be an exception (it's the only country in recent memory I can think of that did a decrease in the time...)
I'm not doubting you here, just curious which countries have formally increased the waiting time.
The revolving door of immigration does not benefit Canada’s
I'd say that there are three main groups here to consider:
Spouses and family-class and similar: allowing a native-born Canadian citizen to reunite with family though, seems to outweigh this. It seems especially cruel to penalize a spouse who is merely following a native-born Canadian outland...
Refugee, protected persons, and similar humanitarian cases: again, I think the humanitarian concerns outweigh this. In fact there is a perverse incentive here - if conditions change back in the home country, a refugee who otherwise might have just wanted to keep the Canadian PR for a few years (in case things suddenly went south) while trying to make it work on a return home, instead must first become a citizen of Canada before being able to securely travel there. Otherwise, even in the event of things going bad after they seemingly went good, that former refugee takes the risk of being stripped of PR and deported right back into that untenable situation when trying to flee back to Canada again.
Edit: If trying to fix a "revolving door" situation I bet fixing things for refugee would effect the biggest change - I highly suspect having Canada stop uniquely stripping PR from refugees for home country visits would lead to most of even the better informed ones to just making the return on Canadian PR only, and if things really have improved for them back home, to them abandoning their PR as they realize they no longer need a way back (and it becomes too onerous to maintain the PR otherwise). Right now the perverse incentive practically requires them to become a full on Canadian citizen before they can even pop back for a quick check of the home country....
Economic class: The one class I'd agree with placing restrictions on. The US has a longstanding policy with conditional PR - you risk losing your PR if you don't fulfill the conditions. Canada's constitution and laws being what they are, I suspect that you'd have to do the implementation up north here a bit differently - instead of offering PR straight away via Express Entry, offer a renewable two year combined open work+study permit (to better match the flexibility provided by true PR), and only on proof of contributions (based on say tax filings) actually grant the PR. If the contribution proof isn't strong enough, then meeting at least a weaker and lower bar allows a grant of a one or two year renewal.
In principle I'm not so much bothered by restrictions on the economic class PRs having to stay until citizenship (as long as we have some wide and reasonable exceptions in place for various humanitarian things), but I can see a practical issue: family-class PRs, in particular spouses, need that freedom. Otherwise, their native born Canadian citizen spouses are the ones who get hurt by not being able to, e.g., seek higher pay outside of Canada (and perhaps build up a private retirement fund to reduce their burden on the government here). But we're required to treat all PRs equally - so if spousal PRs get this freedom, then it has to be given to economic PRs as well.
