The free healthcare only matters if you get permanent residency.. Otherwise, it's just like the world's most expensive insurance plan thanks to the pay cut.
In terms of people in the tech industry I know of who moved to Canada from countries where they get a legitimate pay boost there.. Yes, it makes sense for them. If they never attain permanent residency, at least they made more money than they would have back home for a few years.
However, I've rarely seen one of them turn down a chance to get an additional huge pay boost by moving to the United States afterwards. In fact, it's common for top tier companies to just use Canada as of sort of waiting room or fallback option for folks who are ultimately dreaming of moving to the US to make those higher salaries.
I don't think Canada can compete on pay, but there are certainly people who could get that top pay but would trade it for a chance to live permanently in Canada. I'm guessing those are the sorts of people who have already taken pay cuts to work in highly specialized or speculative areas of science and technology. These are great people to have because they are low risk and high reward in terms of impact on the economy.
Sadly, if you look at any forum for Canadian immigrants you'll find a huge amount of depressing stories about never attaining permanent residency or of grinding for years to game the point system in a way that is soul crushing. It is a broken system on both sides. Let me try to explain what I mean by that.
The European Union does not have a points and lottery based system as it's sole mechanism of entry the way Canada does. If you make above a certain salary, you can get a blue card. You can accumulate time on the blue card in any country in the EU (except for Ireland, which has their own similar system if I recall). After 5 years you can typically apply for permanent residency, in Germany just 2 years so long as you attain B1 German (very practical for usage in daily life and doable in the timeframe without cramming or cutting corners).
The implicit reasoning seems to be that if you can stay out of trouble and earn a good living for 5 years. You're probably worth keeping around. I think a heuristic like that beats a point system which simply encourages gaming behavior anyway.
Check out all of the people cramming French to try to increase their odds in the Canadian lottery for example. Canada probably doesn't get the sorts of genuine francophones they seem to be looking for--cramming for an exam with a verbal component is not the same as fluency--and the people entering the country are doing so by playing arbitrary games instead of living a real life.
And of course the French is just one example, there are also all of the useless courses and degree programs. The points-based lottery encourages and rewards the accumulation of arbitrary points, or in other words, a sort of dissimulation.
I suppose you could argue that Canada is selecting for people who are willing to diligently jump through hoops. If so, that does nothing to address brain drain.