Immobile mids that are viable other than Syndra:
-Lux, high poke, high damage, low cd ult, zone control
-Ziggs, semi-global ult, spammable poke, low cds
-Xerath, his range and damage
-Malzahar, strong mana sustain, impossible to 1v1, high damage
-Viktor, amazing AoE damage and zone control
I don't want to disagree with you, because those champions are 'viable', but they are now where near the strength of syndra.
I also hate to bring LCS / Worlds as a measure of a champions worth, but look at the champions you named vs. syndra.
Syndra: 11 Bans, 14 Picks in 74 Games
Lux: 0 / 0
Ziggs: 0 / 2
Xerath: 0 / 6 (The best one!)
Malzahar: 0 / 0
Viktor: 0 /0
all of these champs dont even add up to syndra's picks and bans.
I'm not saying they are not viable, I main Lux, but i'm confident that Syndras lack of mobility is made up for easily with her huge burst Damage, Great Poke, Great CDs, and just how easy it is to land her Ult for massive damage.