Well for their web based chat the default tags should be what they are now. Only when a user makes the request for certain tags should they request for all, custom or none. This method (as long as it is implemented right) in fact would not break any application currently present in the Twitch Developer Community.
IRC tags are used for passing data to clients and by default should not be sent unless requested by the client as specified in the IRCv3 documentation. For anyone not connecting to Twitch’s IRC servers via their web based chat do not receive any tags unless they request it. This alone disproves your statement about things breaking by defaulting to no tags.
Again, by default clients should not be receiving any tags unless they request it. Unless they’re using their web based chat. Any developer not requesting tags as a capability already won’t be affected, but for those requesting it and are not specifying which tags they need should be defaulted to what is used for the web based chat. As for requesting any/all tags and bloating concerns go, that can be resolved by limiting the amount of tags a developer can request if need be.