With $t:=(y/x)^{1/n}\in[0,1)$, you can rewrite
$$1-t\le(1-t^n)^{1/n}$$ or
$$(1-t)^n\le1-t^n.$$
This is obvious by
$$(1-t)^n\le1-t\le1-t^n.$$
With $t:=(y/x)^{1/n}\in[0,1)$, you can rewrite
$$1-t\le(1-t^n)^{1/n}$$ or
$$(1-t)^n\le1-t^n.$$
This is obvious by
$$(1-t)^n\le1-t\le1-t^n.$$