That’s why they have seasonal events with bonus xp, so people will go back to zones and do things like WBs and dolmens.

Plus, it’s only dead everywhere else now because everyone is in the new hot zone of the Deadlands (no pun intended.)

+1, this is a great idea and is a win for everyone, ZOS included.

Morrowind, for example, is already kind of "base game", but not really, the ESO Base Game comes with Morrowind now, there is no way you can, as a new player, not have access to Vvardenfell.

ZOS should make Vvardenfell (And IC and more) base game, as this would allow for them to introduce quests or content in those zones with new content. For example, Mirri's questline takes you to Stonefalls, rather than Vvardenfell, which is where I think the developers would have ideally liked the story to take you, but it couldn't because not all players have access to Vvardenfell.

The same goes for the Psijic order line, it was restricted to base game zones when it would be better if it were all over the map.

The Deadlands prologue quest takes you to Imperial City, but not the DLC zone, as that is behind DLC (Even though it was free), it would have been better IMO if you went to the actual Imperial City and found the sewer organically rather than having to port into a random sewer section because not all players would have access to the Imperial City.

When ZOS introduces, for example, Ancestral Argonian, it would be great if you could find the motif from chests in Murkmire, but this is unlikely because Murkmire is DLC content that not all players of the new chapter would have access to. It would be better for nearly everyone if Murkmire was just base game, I expect that more crowns will be spent on the new arms pack than sales of Murkmire via the Crown Store for the rest of time.

If ZOS introduces, The Shivering Isles, for example, as a DLC zone there should be a portal to it in Fargrave and that is how you enter the zone, however, that is unlikely to happen as not all players who have bought The Shivering Isles will have access to Fargrave. So just make Fargrave, and the Deadlands, base game. You gave it out for free anyways, why not just make it base game and give new players a better experience.

Personally, I think very few people buy DLC's via the Crown Store and I certainly don't buy ESO+ so I can have access to Murkmire, for example, I buy it for the Craft Bag.
There is already so much to spend crowns on via the crown store, and they are giving away DLC for free all the time, so clearly ZOS doesn't see many people buying DLC's via the Crown Store anyway. Just make the content base game and everyone wins, new players especially, which is good for the game as a whole.

What makes you think they wouldn’t stay dead? Any base game zone after alliance capitals (Greenshade, Shadowfen, Rivenspire and so on) just as dead as old chapters. Nobody would rush killing dragons, geysers and old wb just because it’s suddenly free. Look at Vvardenfell, free dlc chapter and barely anyone outside of city doing quests or bosses, all busy with crafting writs. There is no reason to revisit such zones outside of events, free or not.

I don't see how this is going to fix anything. There are base game zones where it can take time to do dolmens and world bosses because not enough people there are doing that sort of content.