If you rent a flat for 20-40 years, the rent won't change as much as when you move around every few years.
So you might end up with super low rent compared to the rest of the city after 10 years or so.
But I also know a guy from NY who rented a flat for a few hundred bucks decades ago and now subrents it to other people for thousands.
Who is being harmed by a rent-controllee sub-leasing? Literally no one. Because the harm to the owner is already made in the form of rent control.
The owner is harmed, because otherwise they could have gotten a new tenant and raised the rent back up to market rate.
If rent-control didn't have the restriction, then it wouldnt make a dent into rent prices, but it would clearly show that it is a mere transfer from the owner to the rent-controllee.