Do you think that we need to standardize on extensions? The blog post makes it clear that there is room to add additional information to the OAS document ... the question would be where to draw the line? Maybe there could be "groupings" of vendor extensions that would qualify as some additional class of information?
Vendor extensions are flexible / powerful but how to consolidate such that services consuming API specifications can depend on their existence and understand their semantics?
It's a great idea to have standardized extensions. I am a fan of IDE/client-side code checking. With standard Swaggers, we could check if parameter values are valid. However, it may not be possible to check the invocation flows (e.g. /login is required before /checkout) since it's not in the swagger. If they could be provided as extensions in a standard way, existing code checking tools can easily consume them.
In the past we would have formed a standards group, called all interested vendors, and each one would defend their turf, ultimately arriving to a compromise that almost no one used. This has evolved to offering flexibility to experiment and through real usage we may start seeing adoption of certain extensions and at some point it may make sense to adopt them, that makes more sense!