I have no idea why anybody cares about the 15-minute thing. Each person you hire adds thousands of hours to your available labor. So even if I spend 100 hours finding the right candidate, I'm still way ahead. And the better my working environment is, the lower my turnover, in which case I can spend even more.
Is right candidate = person who solves algo questions for 5 hours on a whiteboard?
Imagine we spend an hour per candidate on screening, and we pass 10% of people, and we spend 6 hours on each on site,with 25% of them passing.Finally 50% of candidates accept the offer: This is very typical math, and it ends with over 100 hours per developer hired. For each hire, we also had a declined offer, 6 candidates rejected on site, and 72 failed screenings!!
Given those round, but not really all that far from reality numbers, any increase in screening time will spiral out of control unless some other multipliers change. The one that is most likely is that said company is giving offers to less than half of the people that would have been successful.
Those numbers also show why it's so important for a company to make competitive offers and woo candidates, and why they'd not like it when people interview at 6 places at once: Run the calculations just changing the acceptance rate down to 30% and up to 80%. If you are trying to recruit from a big name university, chances are that your candidate really is talking to a dozen serious SV companies and gets 4 serious offers.
Therefore, while I agree with your sentiment, you can probably see why it's so hard to convince someone running the traditional SV pipeline to make changes, as you are either raising costs or telling them that their current outcomes are a gigantic dumpster fire.
Anybody spending 6 hours per candidate on site can afford to test them on some real work. I'll typically do 2 hours for pair programming, and/or 1 hour for reviewing some existing code. 1-2 hours is also a good amount of time for a joint design session on some real problem.
You're definitely right that we're missing good people. I keep coming across people who didn't get the right job until they invested a bunch of time into learning to beat the Mensa-puzzle interview. So one way to make the numbers better is to find more good people.
I think if we're going to do the math, I also think we need to account for the cost of getting people who are not so good. If we test some thing that we hope correlates with doing the work, we're asking for large downstream costs. The more our interview tests what people actually need for long-term success, the better off we are.
I fully agree with "the more our interview tests what people actually need for long-term success, the better off we are."
I'm more than happy to spend an hour here or there for a phone screen or coding test to show that I understand the basics of data structures and algorithms and that I can use that information and my reasoning ability to solve problems I haven't seen before.