I believe what is capable of happening over time is that the genetics get tired. After years, and stress I believe the genetics slow down often times. I've been running White Widow from the original seed and mother for 4 years now. Some of the guys on here have had strains for 10-20+ years. The better you take care of your plants, I believe the longer the genetics will probably last. I think the harder you are on the plants the harder it will be to keep them going over a long period. They will likely get harder to clone, and perhaps more likely to get diseases. I'm not a botanist yet, so I could be wrong here but I think that's about how it works.
Peace, TC