I would imagine that a lot comes down to maintenance, especially roof and siding maintenance.
If you keep the water out, there's no reason why 2x4 and sheetrock shouldn't last a good long time.
I live in New England, and there's plenty of old post-and-beam construction that's held up for a couple of hundred years. Some houses still have their original interior trim and softwood floors, and if that holds up, I can't think of any reason why 2x4s shouldn't.
But old New England construction lasts because it's maintained. New roofs, new clapboards every 50 years, regular painting, interior repairs and renovations. With land costs what they are in many regions, it's cheaper to take care of a good house rather than just letting it fall apart over 50 years.
The timbers in those old post and beam houses is far different than almost any wood you can find commercially today. Old growth wood from 200 years ago grew slowly, and the rings are compressed due to this. Unless you're building a new P&B home from reclaimed timbers, I doubt it will last 200 years.
