Not sure I understand your point. There is an immigration barrier, as I pointed out - the sponsoree has been denied a visa. The points raised by OP, to wit, cannot leave Canada for a year just to get common law status (the reasons listed) are supporting points as to why the immigration barrier is salient and real.
If not, virtually any potential conjugal case could be thrown out on the loose grounds that "well, the sponsor could just go and live there."
Note, the comparison to a couple that wishes to get married is - generally - the sponsor can travel to the other's country and ... marry them. Or to a third country. Maybe takes a week, a month. Doesn't require the sponsor to move to a different country, change jobs, establish a new household, and drop all of their other family/social commitments in Canada. That's an unrealistic and - dare I say it - discriminatory requirement.
What I don't see in this case is a serious legal impediment to marriage - apart from needing to get a divorce, which is in process in Canada and may take a year or two (or less). It's not forbidden, impossible, or so lengthy that tantamount to impossible.