  The low-lying structures of the midshell $\nu g_{9/2}$ Ni isotopes $^{72}$Ni
and $^{74}$Ni have been investigated at the RIBF facility in RIKEN within the
EURICA collaboration. Previously unobserved low-lying states were accessed for
the first time following $\beta$ decay of the mother nuclei $^{72}$Co and
$^{74}$Co. As a result, we provide a complete picture in terms of the seniority
scheme up to the first $(8^+)$ levels for both nuclei. The experimental results
are compared to shell-model calculations in order to define to what extent the
seniority quantum number is preserved in the first neutron $g_{9/2}$ shell. We
find that the disappearance of the seniority isomerism in the $(8^+_1)$ states
can be explained by a lowering of the seniority-four $(6^+)$ levels as
predicted years ago. For $^{74}$Ni, the internal de-excitation pattern of the
newly observed $(6^+_2)$ state supports a restoration of the normal seniority
ordering up to spin $J=4$. This property, unexplained by the shell-model
calculations, is in agreement with a dominance of the single-particle spherical
regime near $^{78}$Ni.
