It should probably be mentioned that there are edge connectors and
then there are edge connectors.  The cheap ones use a simple leaf
contact that can gradually lose some of its "springiness".  The
better ones use a more complex contact that retain their strength
much longer and have multiple contact points.  See, for example HP US
patent 6152790.
You get what you pay for--unfortunately for all of us, edge
connectors were a common area of cost-cutting.
Cheers,
Chuck