Dreamweaver has always worked this way. Extensions can be updated simply by opening the Extension Manager and selecting the extensions you want to activate again. They do this to prevent incompatible extensions from loading. It's really not a long process at all. Site definitions get migrated instantly. A new cache is created, but that does not take too long. I don't have experience with the user-created commands or dictionaries you are referring to, but if you have a suggestion, post it to their feature request form: https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform .
Adobe programs have never "upgraded" in a sense where previous versions have been removed and a new product was installed over top. This makes troubleshooting new installations much easier whereas something that upgrades software and overwrites new files next to existing files can cause issues down the road which is why many people do a clean install when upgrading an Operating System as opposed to just using the upgrade option.
where did you find the dictionary to be hiding?