Physical ageing caused by 25-years natural storage is studied in glassy As₂₅S₇₅ by conventional DSC technique. The observed effect was recorded as a significant increase in the value of glass transition temperature and endothermic peak area in the vicinity of glass-to-supercooled liquid transition. This result testifies, that marginal values of non-reversible heat flow in temperature-modulated DSC (ΔH_nr), observed for as-prepared (or short-term aged) As_xS_100-x samples within 22.5 < x < 29.5 compositional range [Ping Chen et al 2008 Phys. Rev. B78 224208], can significantly increase after longer period of physical ageing. Owing to these and previous results on collapse of the reversibility window in As_xSe_100-x glasses after long-term physical ageing, we have concluded that marginality of non-reversible heat flow is not a sufficient criterion for express identification of the optimally-constrained self-organized phases in chalcogenide glasses. To make a conclusion on the marginality of ΔH_nr, the long-term component of physical ageing should be taken into account. A comparison between conventional or temperature-modulated DSC scans recorded for long-term aged and rejuvenated glass samples is considered as a suitable criterion for physical ageing in glasses and, therefore, for the possibility of their network to be optimally-constrained.