That 50 year claim always seemed wild to me. Shellac is an amazing sealer, but it's made from bug resin and alcohol. It can get brittle and fail with big humidity changes over time. I've seen it crack and let tannin bleed through on old furniture after maybe 20 years, not 50. It's good, but not a forever fix.
Tried shellac on a knotty pine table that kept bleeding through, and it worked great for about 15 years before I needed to redo it. What kind of wood are you sealing?
It reminds me of how many old claims from trade magazines get repeated as fact. We trust the printed word without seeing real proof over time. It's a good lesson to question even the most official sounding advice.