Sure. Core game mechanics. The way that a Soldier interacts with the game and the tactics deployed equate to the games essence. Gameplay on the whole has always felt pretty balanced to me. The way I envision your suggestion being implemented in an in-game situation would amount to a drastic change in gameplay, particularly in CTF. Allow me to elaborate...
A) As I'm sure you're aware, being good at using existing movements is an integral aspect of being good at Soldat. Being able to both retreat and return fire whilst evading enemy fire i.e. switching to & from supine, AS WELL as maintaining momentum I believe would create an unfair advantage. Currently whilst retreating/flag running you can either opt to return fire AND maintain momentum, leaving yourself open to incoming fire OR you can opt to evade attacks (prone switching) AND return fire sacrificing momentum OR you can opt to gain momentum AND evade attacks (prone+jet/switching) sacrificing ability to return fire. These three variables offer good strategic gameplay and overall balance. I think that if your idea was used everyone retreating/flag running would simply opt to supine jet away because you wouldn't have to sacrifice any abilities - speed - evasion - defence.
B) In a combat situation where a Soldier approaches you, you could effectively direct incoming fire at your feet taking less damage whilst still returning fire. Similar to point A and to what L[0ne]R said this would create an unfair trade off.
C) I use X for crouch and C for prone. E has always felt somewhat awkward for me so in my personal circumstance I would have nowhere to map this feature.
D) I went into this a little deeper than I had intended to. Above all I am very stubborn when it comes to Soldat. I don't like change. Especially not such drastic changes to the core game mechanics. My initial reaction to your suggestion was no, no and more no. Simply based on the fact that it is 'change' but I also think the points I make are valid ones. Though I do see some merit in your suggestion in that it could look quite cool to have Soldiers supine off of structures or away from explosions but I think the novelty would wear of quickly and we'd be left with what is ultimately a huge game changer.
I hope that satisfies your query, Sir.
Thanks for elaborating, and I appreciate your candor in admitting that you don't like change.
To your points:
A) Those that are better at Soldat already have an advantage over other players; they are already able to return fire, evade attacks, and maintain momentum better than other players. Since this move will be available to all players, equally benefiting all players, those aforementioned good players will still have that same advantage.
B) Again, like A, an advantage for everyone is an advantage for no one. Both players can go into supine so the feet take some of the incoming fire.
C) The suggestion's being going the way of having prone cancel replaced with supine position; so no added key would be needed. However, if a separate key was needed, most players use the default 's' for crouch and have 'c' unmapped. You're a rarity, darling; in your case, you could map it on 'z' or even the default 's'.
D) To me, this is your truest and most valid reason and, like I said, I appreciate your candor. You seem like a reasonable guy; would you be willing to try out the move in a beta to both test the validity of your other points and also challenge your stubbornness to change and see whether it actually improves gameplay, worsens it, or just changes it to something different?