As far as I remember, 10 years was the default setting when creating a keystore in Eclipse. I think, about 5 years ago, Android development got kind of mainstream so I expect a lot of apps to be un-updatable in the Play Store in 5 years.
So far, I haven't found a solution on how to "prolong" a certificate's lifetime. Is there one? Will Google implement a way to set up your app with a new certificate? Of course, my scenario doesn't affect TOO many apps but I think there will be a considerable amount of SO questions on just that in 3-5 years.
Any ideas?
Google Play has always required a certificate to be valid for at least 25 years.
Starting with minSdkVersion 28, it's possible to change certificates. So, app developers will probably want to drop support for older versions about a year before their certificates expire. Then, for the remainder of the old certificate's lifetime, updates will be signed with both old and new certificates, and then only with the new certificate after that.