I agree slinger image should be redesigned - but someone has to do it, and apparently no one has time/skills/idea. There is already topic about that.
Talking about reality - how do You figure how much training is needed for proper sling use? I only found in google something like this
The sling requires enormous skill, one that can generally only be obtained with training from childhood (Hawkins, 1847; Korfmann, 1973; Wise, 1976; Ferrill, 1985)
Most slinger units used in history were specific tribes, so it actually proves requirement of skills.
There is also huge balancing issue - making unit twice as fast in training is too much to just rebalance. Especially making any unit 1 turn.
Making any 1 turn unit is dangerous, as 1-turners can be used as tc occupier (there is many scenarios/maps having few TC just beside each other, randomly generated map can have TC quite close too, especially on tiny maps) and additionally could be amassed quickly.
Any ranged unit in mass can be very dangerous.
The current one has it's role - deals 12 damage/turn to melee units and survives 1 light cavalry attack.
The only reason it's not used more often is his miss chance - it doesn't make him much weaker, but random generator can be cruel and very, very annoying (missing all four attacks in 2 turns against skirmisher with 1hp left).