  We show that in the absence of trilinear terms in the scalar potential of
Georgi-Machacek model, heavy charged scalars do not necessarily decouple from
the $h \to \gamma \gamma$ decay amplitude. In such scenarios, measurement of
the Higgs to diphoton signal strength can place stringent constraints in the
parameter space. Using the projected precisions at the High Luminosity LHC
(HL-LHC) and the ILC, we find that the upper bound on the triplet vacuum
expectation value can be as low as 10 GeV. We also found that when combined
with the theoretical constraints from perturbative unitarity and stability,
such variants may be ruled out altogether.
