  We study the electronic phase diagram of the excitonic insulator candidates
Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$ [x=0, ... ,1] using Raman spectroscopy. Critical
excitonic fluctuations are observed, that diminish with $x$ and ultimately
shift to high energies, characteristic of a quantum phase transition.
Nonetheless, a symmetry-breaking transition at finite temperatures is detected
for all $x$, exposing a cooperating lattice instability that takes over for
large $x$. Our study reveals a failed excitonic quantum phase transition,
masked by a preemptive structural order.
