[swift-evolution] [Review] SE-0117: Default classes to be non-subclassable publicly

[Tino Heth]

> Of course it can be done either way.  But there are significant ecosystem robustness advantages to making sealed the default and comparatively few downsides.  Most libraries are open source (so can be modified directly or via PR if necessary)
First:
The claim about robustness sounds like a fact, despite being just an opinion (feel free to correct me if you have any evidence at all). We should stay honest with our predictions.
Second:
Do you really believe there will be positive impact on open-source libraries?
My forecast is that closed by default will dramatically increase trivial pull request where developers ask for unsealing so that they can do as they like… and I've no idea why somebody could come up with the idea that forking is desirable.