  There are two seemingly unrelated puzzles about the cuprate superconductors.
The first puzzle concerns the strong non-BCS behavior around $x_{c}$, the end
point of the superconducting dome on the overdoped side, where the cuparte is
believed to be well described by the fermi liquid theory. This is the most
evident in the observed $\rho_{s}(0)-T_{c}$ scaling and the large amount of
uncondensed optical spectral weight at low energy. The second puzzle concerns
the remarkable robustness of the d-wave pairing against the inevitable disorder
effect in such a doped system, which is also totally unexpected from the
conventional BCS picture. Here we show that these two puzzles are deeply
connected to the origin of a third puzzle about the cuprate superconductors,
namely, the mysterious quantum critical behavior observed around $x^{*}$, the
so-called pseudogap end point. Through a systematic variational optimization of
the disordered 2D $t-J$ model from the resonating valence bond(RVB)
perspective, we find that the d-wave pairing in this model is remarkably more
robust against the disorder effect than that in a conventional d-wave BCS
superconductor. We find that such remarkable robustness can be attributed to
the spin-charge separation mechanism in the RVB picture, through which the
d-wave RVB pairing of the charge neutral spinons becomes essentially immune to
the impurity potential except for the secondary effect related to the
modulation of the local doping level by the disorder. We propose that there
exists a Mott transition at $x^{*}$, where the RVB pairing in the underdoped
regime is transmuted into the increasingly more BCS-like pairing for $x>x^{*}$,
whose increasing fragility against the disorder effect leads to the non-BCS
behavior and the ultimate suppression of superconductivity around $x_{c}$.
