Hmm that is a downside that they may degrade over time. Like 10 years or are you saying less?
You can't really put a number on that. It depends on how the customer plays the speakers and the design implementation.
Low crossover point, low order electrical filters and how loud the customer plays them will determine this. The problem is stretch of the ribbon from the lower frequencies and volume has in impact on that. Certainly a ribbon is not a good choice for a high power loudspeaker.
Again this goes to making the implementation difficult, as acoustic roll off of the selected drivers is really the determining factor in the crossover point and to quite a large extent the electrical order of the crossover.
I would say that I try to use the acoustic roll off of the drivers as much as possible to minimize time shifts. However the acoustic roll off does not protect the ribbon, as the acoustic roll off occurs because of failing output as the frequency drops, but only mechanics limit excursion, which puts the ribbon on stretch.
All these factors tend to dissuade me, as longevity is always a primary goal in what I design.