Context. Erupting flux ropes play a crucial role in powering a wide range of solar transients, including flares, jets, and coronal mass ejections. These events are driven by the release of stored magnetic energy, facilitated by the shear in complex magnetic topologies. However, the mechanisms governing the formation and eruption of flux ropes, particularly the role of magnetic shear distribution in coronal arcades, are not fully understood. Aims. We investigate how the spatial distribution of magnetic shear along coronal arcades influences the formation and evolution of eruptive flux ropes, with a focus on the evolution of mean shear during different phases of the eruption process. Methods. We employed 2.5D resistive magnetohydrodynamic (MHD) simulations incorporating nonadiabatic effects of optically thin radiative losses, magnetic field-aligned thermal conduction, and spatially varying background heating in order to realistically model the coronal environment. A stratified solar atmosphere under gravity was initialized with a non-force-free field comprising sheared arcades. We studied two different cases by varying the initial shear to analyze their resulting dynamics and the possibility of flux rope formation and eruptions. Results. Our results show that strong initial magnetic shear leads to spontaneous flux rope formation and eruption via magnetic reconnection, driven by the Lorentz force. The persistence and distribution of shear along the arcades are crucial in determining the formation and onset of flux rope instabilities. The shear distribution infers the non-potentiality distributed along arcades and demonstrates its relevance in identifying sites prone to eruptive activity. We have explored the evolution of mean shear and the relative strength between guide field and reconnection field during the pre- and post-eruption phases, with implications of bulk heating for the "hot onset" phenomena in flares, and particle acceleration. In contrast, the weaker shear case does not lead to the formation of any flux ropes. Conclusions. The spatial distribution of magnetic shear and its evolution and mean shear play a decisive role in the dynamics of flux rope formation and eruption. Our findings highlight the limitations of relying solely on footpoint shear and underscore the need for coronal-scale diagnostics. These results are relevant for understanding eruptive onset conditions and can promote a better interpretation of coronal observations from current and future missions.