When the low late summer numbers hit zero those cannot go any lower. This in itself is a reason for expecting the rate of decline to start to level off.
This in my view is an understanding of the system that forgets about the physics, ie the ocean enthalpy. In a way you are saying that as ocean heats to the point of ice–melt, it cannot heat further. We all know that to be wrong. Water can easily heat to 100 C before it starts boiling and turns into gas (vapor).
In an Arctic context there is a continuum: Ocean continues to heat, it gets to keep that heat into the next day, and thus everything continues, it doesn't drop dead the day Summer ice melts.
You are, in other words, confused by the number zero. This implies you are looking exclusively at the ice, and not at the ocean melting it. If you look instead at the warming ocean, there is no number zero and no brick–wall to crash into. Instead steady warming continues.
A year passed. Winter changed into Spring. Spring changed into Summer. Summer changed back into Winter. And Winter gave Spring and Summer a miss and went straight on into Autumn. Until one day...
The extra albedo effects of earlier and longer–lasting meltdowns will, together with energy saved (no more work needed to melt ice during that period) ensure a very aggressive expansion of the meltdown period in both directions. And that is without considering all the other strong positive feedbacks (melting onshore and offshore permafrost, snow–cover albedo, darkening from Boreal wildfires, fierce cyclones etc).
In a One Decade scenario (first open day to first open year) the open sea period will on average increase by 1.2 months per year. That gives this hypothetical timetable for the final melt:
2021: 1 day meltdown.
2022: 1.2 months meltdown.
2023: 2.4 months meltdown.
2024: 3.6 months meltdown.
2025: 4.8 months meltdown.
2026: 6 months meltdown.
2027: 7.2 months meltdown.
2028: 8.4 months meltdown.
2029: 9.6 months meltdown.
2030: 10.8 months meltdown.
2031: 12 months meltdown.
NB! With positive feedbacks and abrupt events added, things will probably move a lot faster, though.