Poly(lactide) (PLA) is a promising biodegradable polymer with potential applications in single-use packaging. However, its use is limited by brittleness, and its biodegradability is restricted to industrial compost conditions due in part to an elevated glass transition temperature (Tg). We previously showed that addition of a poly(ethylene-oxide)-block-poly(butylene oxide) diblock copolymer (PEO-PBO) forms macrophase-separated rubbery domains in PLA that can impart significant toughness at only 5 wt %. This work demonstrates that PEO-PBO/PLA blends exhibit substantial toughness for at least nine months, beyond the average lifetime of single-use packaging, even amidst oxidative degradation of PEO-PBO into oligomeric products. Due to the glassy nature of the PLA matrix, these degradation products are confined to macrophase-separated domains, and the blend morphology is preserved. However, modest thermal annealing (∼60 °C) causes these domains to rapidly reduce in area fraction and size from migration and solubilization of the PEO-PBO degradation products into PLA, which plasticizes PLA and reduces the blend Tg. As a result, aged PEO-PBO/PLA blends degrade in just under half the time of similarly aged neat PLA when submerged in artificial seawater at 50 °C. This surprising combination of properties addresses two of PLA's most significant limitations with a single additive by (1) toughening the PLA during its useful lifetime and then (2) accelerating its degradation rate by heat-triggered plasticization when exposed to elevated temperatures at end-of-life, such as those of industrial (or even home) compost.