In many jurisdictions, including New York, the law requires that landlords allow subleases. Tenants have a lot more rights than you might think.
In NYC, it's usually required for a subletter to take over the remainder of a lease and then have the option of signing a new lease. The option to simply immediately break the old lease and start a new one usually doesn't exist. (Or if it does, it's only with a penalty for the lease-breaker.)
I don't know why this is, however -- if it's for administrative convenience, legal reasons, or financial reasons. But your idea that "the landlord can agree to the transfer of a lease", well it's true they can, but as a general rule they won't.
> We were able to prove that we could make the financials work so long as we were able to fill the apartment within around 45 days of taking it over.
They're prescreening apartments/leases so they're not going to take on an apartment they can't turn around in a month. And honestly, modeling NYC rental supply and demand according to a number of factors (neighborhood, price, condition, amenities, etc.) is pretty straightforward. It's a relatively liquid market.
And like I said, landlords generally have explicit rules about tenant qualifications. They're not going to reject tenants on a whim. Why would they ever say no to a qualified tenant? That's like McDonald's refusing to sell you a quarter pounder.
There's nothing about this that seems obviously unsustainable at all.
I agree with the rest of your comment, but in my experience landlords in hot markets can be pretty capricious. If they know they'll have a steady stream of applicants, many will definitely reject qualified tenants based on personal whims or to hold out for someone they see as "more" qualified or more likely to stay long term. That said, as long as the apartment still gets turned around quickly then it's not really a threat to this business model.
So a better way to phrase the question would be "Why would they ever say no to all qualified prospective tenants?"
Yes they'll sometimes pick the most qualified applicant, but only if you submit your application the same day or same weekend, which is often the case with open houses. It really depends on the landlord however -- for a lot, it's literally the first qualified person to apply, which is why you want to be the first person to the open house and have all your documents ready to apply on the spot, since it might be gone 30 minutes later.
I've never heard of a landlord not allowing a subletter to convert to a full lease upon original lease expiration.
I mean, the alternative is to forego a month or two of rent while you find a new tenant. Unless there's a horrible problem with the existing subletter's credit, but then they probably wouldn't have gotten the sublease in the first place.
I'm not saying it's never happened, but it's going to be rare. I don't really see anything unsustainable or mispriced about this at all. There are already other companies doing it as well in NYC, e.g.:
https://doorkee.com/