Hmm, I forgot about Mimics, and I do not think I have ever built an Aquebusier.

I should have been more specific, when I said "loses something", I had meant a magic ability or promotion (not defensive and attack bonuses). In my mind, a Divided Soul losing sever soul is like a Cultist losing tsunami when it upgrades to Speaker to get summon Kraken (or something similar).
If the recon line having sever soul is considered too powerful, as Blackmantle suggests, then I really would like to see DSs not be replaced by Rangers. But I am not completely convinced that a Ghost or Shadow with sever soul would be game breaking, or even overpowered. I think the benefits it provides to playability of the Sidar are worth it.
In my last game as the Sidar, I was amazed at how well the surival mechanic of the DS worked to help me get Wanes. In the past, the best way to get Wanes was to build a lot of Adepts and wait, or build the Altar of Luonnotar and make a lot of priests. For me, waiting on Adepts is really boring (and I
prefer non-combat wins normally o_O ), and making the Altar seems out of character for the Sidar (what do they care about a one true God, and I usually go CoE anyway). Giving the recon line (or any units) a helping hand in escaping from combat with valuable experience points was a fun, active way of making wanes. I think expanding this ability beyond early to mid-game would keep the Sidar play style interesting for the player. For units that can be invisible or gain HN, the sever soul ability helps them get back to their city faster so you can build a new unit and start the process over again.
The fact that DSs can hop over mountain chains, while great, only really applies to one map type. I do not think that should be a make or break point.
In short, letting DSs upgrade and keep their sever soul ability makes getting wanes a more active experience for the player, less frustrating, and makes non-adept/priest strategies more viable.