We have no empirical evidence measuring the behavior of light waves over large distances and time. If it takes ten billion years for the effect to occur, it might be too small for us to currently measure. Our theories are incomplete, this much is known. If our expansion is to continually increase, our time will slow down, our mass will increase and the rest of the Universe will be essentially accelerated black holes. If it is actual speed we are measuring.
If we were already inside an event horizon, it would explain a lot of what is observed (lensing, the “wall of galaxies”, etc). :’)