In addition to that, I'd say the Shard Travel evidence that is in the Main Story is itself the evidence against Shard Travel happening in general. Or at least, Shard Travel for the Scions (and the extreme majority of characters). There's a few requirements a person needs for Shard Travel (as presented in the Main Story) and they are pretty exclusive.

The first requirement needed is the Echo itself, specifically the Power to Transcend Worlds, as well as the side ability that seems to come with/because of it, being able to hold your soul together without a body. Very, very few people have this.
The second requirement needed is the "renunciation of the flesh". In other words, a person forcibly separates their soul from their material body and rely on the Echo to not dissipate into the Lifestream/Aetherial Sea. This keeps a soul from properly dying by not returning it to the Lifestream. The only thing is, this is a soul. Interacting with the physical world is impossible at this point. In order to do so, a soul needs to posses a body. This lets them have a physical vessel so they can interact with the physical world. It also seems to make it easier to "resist" being pulled into the Lifestream.
The third requirement needed is a Crystal of Darkness/Light. This is what lets someone get to the place in between the Shards so they can travel to the other Shards. It should be noted that it is possible to posses bodies without a Crystal of Light/Darkness. However, not having one means a soul can't flee to the Space Between the Shards. The Ascian Overlords and the Warriors of Darkness all have Crystals of Darkness/Light. The lower level Ascians (and most Echo users it seems) do not have these. It is possible to kill someone possessing a body if the possessor does not have a Crystal of Darkness/Light and there are not any dead bodies that can be possessed.

This makes Shard Travel possible for only a very small group of people. In fact, the only person we know of who has a Crystal of Light now is us. We're pretty much the only person who isn't an Ascian that could Shard Travel. The only problem I have with this is that the expansion trailer and the expansion art seems to be focused on the Scions and they don't even have the Echo. So if they are going to Shard Travel, it's going to be for reasons that probably are not in the game yet. I'm not going to say it couldn't happen, just that all we know about Shard Travel so far makes it seem like Shard Travel is something we'll never do as it would mean leaving behind pretty much everyone we know. They would have to do a lot of lore finagling to make Shard Travel for people without a Crystal of Light possible.