Search engines (including Google) index neither "404 Not Found" URLs nor "410 Gone" URLs. From an indexing perspective using 410 as the default instead of 404 isn't going to be any different.
The biggest difference between the two statuses is how quickly Google removes a page from the index when it no longer exists:
- When Google encounters a 404 error for a page that used to exist and is currently indexed, it waits 24 hours before removing it from the search results. It is fairly common for websites to accidentally remove pages (or even the whole site) and for Google to get a 404 error temporarily. Because of this, Google has a grace period.
- When Google encounters a 410 error in a similar situation, it removes the URL from its search index immediately. Google views a 410 error as a deliberate attempt to remove a page.
Thus using 404 as the default error but 410 for pages that are intentionally removed gives you the greatest power. It offers protection for your site when you make accidental removals but allows you quickly remove pages from the search index by setting a 410 error for them when they are deleted.
A 410 error means "this page was here but is now gone" and a 404 error means "this page could not be found." 410 errors are a subset of 404 errors and the semantics of a 410 status would not be appropriate for a URL that has never had content on it.
I would not recommend using a 410 error as a default error instead of "404 Not Found." The semantics of 410 aren't right for the default, and using 404 as the default gives your website some SEO protection from accidental deletions.