Earlier quoted context omitted.
1. Often, legally, firing people is harder than hiring. 2. Hire more, fire fast sounds good on paper but will destroy morale as the team identity will be constantly influx. 3. Engineers will spend less time onboarding newer engineers to the detriment of everyone because well, they might not just last.
Basically everywhere in NA has allowances for probationary periods with at-will employment. Moreover, prospective hires could start on contract. Morale is improved when team members feel like they have some control over their membership. It seems like you're assuming that a great many people would be fired; why so? In my experience, having worked for several such companies, firing is rare because even minimal filters…
For one thing, it's a huge amount of uncertainty to put on the potential hire, which means you'll be strongly biasing your hiring pool towards people who don't have better options. It feels like in an effort to reduce the interview time, you've effectively expanded it to multiple months.
Second, how do you calibrate the "default" option? Is the expectation that everyone on a probation period will be hired unless proven otherwise, or is it that most will be let go unless actively vouched for? Is that expectation clear across the team? If there's a mismatch across team members, you have big problems. People who want to keep a prospect will be annoyed if they're let go, and vice versa. People who need to work with a prospect will have to figure out whether it's safe to actually trust them with anything - on the one hand, you have to give them enough work to prove themselves; on the other hand, you can't give them anything too important or with too long a horizon, because they might be gone before they get to launch.
And finally - how long does that trial period need to go on to be useful? The goal in designing a hiring process is to get a reasonable level of precision/recall (different companies will balance differently between those) with a reasonable level of investment. If you've increased investment without increasing precision, then you've done something wrong. Given how long it takes to ramp up a new hire and have them actually be productive, I don't think you're going to learn much in a ~6 month probationary period that you couldn't already tell in a day of interviews.