A class which is extensible implements the feature that it can be extended — that’s a feature like any other feature of the class, and should be treated like one, no different from a method. All features should be thought through carefully to ensure that they meet the goals of the customer using the feature. Features need to be designed, implemented, reviewed for security problems, debugged, documented and maintained.
All that costs effort, and effort usually requires the outlay of money. Whose money are you spending? They might have an opinion on whether you should do this feature or not.
Basically, you have three choices:
1) Spend the money to do the feature so that you have confidence that it is correct, robust, secure and meets user needs.
2) Do none of the above but ship the feature anyway and hope that shipping an undesigned, rapidly implemented, untested, undocumented, unmaintained feature with unknown security risks doesn’t harm you, your employer or your customers.
3) Seal the class. Unseal it later if you find that (1) was the right choice.
I say that (3) is good value for the money. I always seal every class I write that was not designed for extensibility.