It's not a crown of immortality. It reflects your natural lifespan, unless it de-ages you, then it reflects your relative lifespan from that age onwards. The fact that strong magic power rejuvenates you as crown is put upon you is based on the super crown being a stacking powerup that can function same time as power star. It replaces only effects like fire flower, not of super shroom or the stars, thus it does not eliminate effects of power boosters, only "transmutative" ones. So in my canon it's effects stack on top of existing power levels, and may even be enhanced by high power levels, as it also de-aged bowser to peach-level age. Bowser has magical powers, not on par with Kamek, but still enough to enable rejuvenation. Meanwhile a common plebian like Toadette only reflects her present age.