I think that it's unlikely that those who got CRS in the 420 region or below would be deemed undesirable by IRCC. The bare minimum is set outside of the CRS system by ensuring that, for example, FSW applicants must score 67/100 on the FSW points matrix. As long as the minimum requirements are met, I don't see why IRCC would have any interest in stopping people who are eligible from becoming PRs.
In terms of raw human capital CRS scores - compare the situation we have now to the situation we were in prior to November when IRCC were quite happy giving ITAs to anyone who had a job offer - irrespective of their CRS score. There were a lot of people who had CRS of maybe only 300 but their job offer got them bumped up to 900. IRCC didn't have a problem with this per se, although one side effect of that was that they saw a much larger than expected proportion of ITAs going to people with experience as fast food supervisors and cooks. That has been fixed by ensuring that those who are able to get a job offer will get a boost, but not such a boost that it would automatically give you an ITA. They didn't artificially restrict CRS back then and there's no reason they would start now.
Furthermore, 420+ isn't actually all that easy of a target for a lot of people. Consider those who have graduated from university less than three years ago as a good example. You would expect that young people with good education and perfect English would be just the kind of people that Canada wants - but no: someone applying for FSW with top IELTS, one year of experience, under 30 and a bachelors gets only 416 points. Likewise, someone very experienced in their field at age 29, with ten years of work experience, who has perfect English and a two year program at college as education gets only 419 CRS points. Both of these hypothetical people are highly desirable but have relatively low CRS. They will also not be able to increase their CRS without bettering their education or gaining more experience (short of getting PNP or a job offer).
Canada needs a good mix of people with varying levels of ability. I think that the CRS system combined with PNP and extra points for job offers is a fair system. With that being the case, I don't see any rationale that IRCC would have for artificially limiting CRS scores. Quite the opposite, as I've demonstrated above - it's in the interest of Canada to have people with lower human capital scores come in as well: many will be very valuable to the Canadian economy in the future!
