The reason the standard limit on a reveg is usually talked about as being 3 times is due to issues related to being root bound, root rot, ph level going way off, nutrient deficiencies and any other issue you can imagine with the roots after a long life. At some point for whatever reason there will be a point of diminishing returns where you're just better off starting with new plants again. How well you take care of them, what media they're in, pot size limts, etc all play into how long you can realistically expect to keep that up.