I'm not sure what you're trying to ask. I think you're asking if an unlined
pond will eventually drain out. The answer is it depends on so many things
as you pointed out. Depth of water table, soil type, porosity of soil,
Obviously natural ponds and lakes are proof, that some can exist
perpetually, though they are often fed by rain run off, rivers or just an
incredibly shallow watertable.

Water table depth , porousness of the soil, evaporation rate, and rainfall
all impact the water level of an unlined pond. Contrary to your assessment,
these factors all play heavily into the viability of an unlined pond.
Consider that some ponders in new mexico and arizona have reported a water
loss of 1-2 inches per day from evaporation in the summer.

Natural ponds also experience similar cycles, some years they dry out
completely, other years the water level recedes drastically, other years
they flood over their banks. Call your local water district, they'll be able
to answer these questions as they relate specifically to your area.

In my area, the water district/municipality maintains an artificial pond
near my house that are designed to help refill the ground water supply. I
bet they know exactly the rate at which the water level fluctuates, and even
exactly where the water goes, evaporation or absorption into the soil.

Good luck.