Like most others I have both types, mechanical and electronic/computerized. I have only had 2 SM that ever failed. Both were my vintage mechanical ones, a Singer 648 and a Brother 622. Sadly, they both died of cracked gears in the last few years. They each gave decades of service to me and their previous owners, so not complaining at all.
I have had computerized/electronic machines since 1990 or so, and happily, none of those have ever needed work or failed in any way. I don't expect anything near the 50 year lifespan of those 2 older ones, but amazingly 15-20 years actually seems fairly common, as there are loads of 1990-2000's machines still sewing along, including my own. I would not pay much for a 20 year old one, or ask much for one I was selling, as much of its lifespan has likely been used up, but many are definitely still quietly humming along, so this is probably quite variable.
For me, SM seem to outlast all other household devices, from dishwashers and microwaves to laptops and cell phones. Yay!