"Endless hot water" is real — right up until simultaneous demand exceeds the unit's flow-rate ceiling. A tank never has that ceiling, but it has a very different one.
A tank water heater and a tankless water heater solve the same problem — deliver hot water on demand — by drawing on two completely different limited resources. A tank stores a fixed volume of already-heated water and constantly loses a little of that heat to the surroundings even when nobody is drawing water (standby loss); once the stored volume is drawn down faster than the burner or element can reheat it (its recovery rate), the water coming out of the tap gets cooler. A tankless unit stores nothing — it fires its heat exchanger only when water is flowing and heats that water as it passes through, so there's no standby loss and, in principle, no volume to run out of. But a heat exchanger can only add so much heat to so much water per minute, which means a tankless unit has its own hard ceiling: its rated flow rate (GPM) at the temperature rise the job actually needs.