Bio-based semicrystalline polylactide (PLA) has a growing value as a substitute for fossil-based polyesters in technical applications and as a biocompatible material in medicine. Poor toughness is generally recognized as a limitation for the expansion of PLA usage in the applications that require elastic-plastic deformations at high stress levels. An identified research challenge is to develop new insights and approaches to guide the mechanical properties of PLA to a level that will make it suitable for industrial usage as well as to programm its degradation rate with regards to the particular environmental/application conditions. Presented research reveals that under certain environmental aging conditions melt-spun highly crystalline neat PLA monofilaments demonstrate a long-term preservation of the toughness, outlasting the hydrolytic degradation. Environmentally triggered structural changes and hydrolytic degradation of neat PLA and blended PLA monofilaments have been evaluated by analysis of their crystalline structure, thermal and mechanical properties as well as their long-term relaxation behavior using a self-developed model. A self-developed model was elaborated to predict the long-term mechanical behavior of the fibers. The mechanism behind the observed durability of PLA material is presumably attributed to the relaxation of the confined amorphous phase presumably as a result of local chain scission. Presented results offer possible design strategies toward tough neat PLA materials for sustainable technologies.