Phenomenal conservatism isn’t foundationally justified, the justification is more pragmatic—without PC, you have no starting point , and can’t say anything about anything.
PC is one possible starting point, but there are others. For example, axiomatic mathematics allows you to say that 2+2=4, without the justification for that being “it seems that 2+2=4”. As another example, you could assume anti-PC—that however the world seems to you, the truth must be the opposite. So “without PC, you have no starting point” is not a reasonable justification for PC, even from a pragmatic point of view.
PC is one possible starting point, but there are others. For example, axiomatic mathematics allows you to say that 2+2=4, without the justification for that being “it seems that 2+2=4”. As another example, you could assume anti-PC—that however the world seems to you, the truth must be the opposite. So “without PC, you have no starting point” is not a reasonable justification for PC, even from a pragmatic point of view.