Don't know if this has been suggested, but moving back-stab onto the scoundrel skill might be a possibility too. It would still be limited to daggers and still require you to "learn" it.
I agree that as a talent it is purely a tax for rogues, and a slight nerf would be much better than the current arbitrary tax.
Well it already is kind of on the scoundrel skill by increasing crit multiplier. However, right now I'm not sure how much better Scoundrel is than just investing in Dual Wield. DW boosts all damage, while Scoundrel just boosts crits. The increased movement is probably the main benefit of scoundrel over DW right now (potentially +50% movement is better than +10% dodge imo).
So they could slightly decrease the base backstab or even general crit multiplier but slightly increase the crit multiplier gain from rogue skills. The default crit multiplier is just +100% damage, right? They could potentially drop it to +75%, and boost the scoundrel crit multiplier to +8% or so. Maybe very slightly increase dagger base damage with the reduction in base crit multipler damage to counteract the reduction in damage (only with a full 10 points with +8% crit scoundrel base would you deal more damage than +5% crit multipler scoundrel with 100% base.)
This would nerf all non-scoundrel crit builds of course, but incentivize investment in scoundrel to hybridize as well, which is interesting. Though it's a bunch of tweaking for a relatively small change, so I don't know if it's worth it in the end. The essential downside of backstabbing should be its positional requirement, not a talent tax or even requirement for a certain ability to be even remotely viable. Non-rogues should generally lack the movement and skills to reliably position themselves for backstabs without spending more AP than you gain in crit damage.
I think rogues are in a pretty decent state of balance already, the most reliable high single target physical damage dealer in the game that can get into bad situations if making poor decisions. I don't think removing the talent tax really needs to be offset by a balance change at this point, though in later game maybe rogues will get too good or perhaps worse, I dunno.