The Moog Grandmother is an analog synth often kept long-term because its character and hands-on workflow don't become "obsolete" the way some digital gear can. Failures are commonly mechanical interface parts: key contacts, pots, and jacks-especially if the instrument is gigged or transported frequently.
Repair under the threshold is usually sensible because restoring a known instrument preserves your sound and avoids the uncertainty of a replacement unit. Replacement becomes more rational when repairs involve repeated board-level faults, persistent power issues, or major damage from liquid/impact, since analog calibration plus parts labor can escalate.
Also consider your use case: a studio-only instrument with gentle handling usually justifies repair, while a road-worn unit with recurring connector issues may be better replaced for reliability.