If you impose mandatory higher standards for pay, benefits and training on temp employment agencies, a great many of these jobs will simply cease to exist. Given they're employing some of the most resource-challenged people, such a social intervention is liable to cause far more harm than good. Perfect is the enemy of the good.
Not sure if you read the article, but no one is talking about a higher standards for pay and benefits. You're literally making generic libertarian points. The only direct criticism is that they don't enforce their own safety guidelines which leads to the death of their workers. A great many of these jobs can continue to exist even if their managers simply request those with loose garbs or improper safety shoes, etc.…
https://www.thestar.com/news/queenspark/2017/09/11/temp-work...
Publicity of the working conditions and pay of low-skilled workers invariably leads to calls for more stringent mandatory standards, as if you can outlaw poverty and low-living standards.
>>Unless the onus of taking on such check was so great that they suddenly decide to put all their money into R&D, these jobs will continue to exist.
The effect is on the margins. No single mandate will have a significant impact, but each one adds costs, and in the aggregate, they have a significant effect on economic growth, which is a compounding effect that translates to massive long-term effects.