Only if you're residing in Canada. If you're residing abroad, you only pay taxes on Canadian income. I do not know about pensions.
Yes, but 2 things to clarify: 1- Their children are Canadian so they are entitled to education. 2- As for the 2 out of 5 yrs, that's just to keep the PR, not to receive healthcare or any benefits. And to be a citizen they need at least 3 of 5 and the intent to stay after citizenship (and citizenship processing takes a year easy) so basically they're stuck for 5 years of physical presence.
A Canadian meanwhile just flees right after their subsidized education and make loads of money in the US or the Gulf, without paying a penny back. That's especially true of "Canadian" Canadians (as this discussion shows, many Canadians do not see immigrants as equally Canadian entitled to all rights). But as you pointed out, it's not sustainable. In fact, the only way it's still going on is through more and more immigration. You'll see that plain clear if/when Canadians elect their own conservative populist (Max?). Entitlements & benefits need an increasing tax base, and if the workforce isn't expanding accordingly, then the rates will.