You forget however that AT nade is not 100% success anymore. Against Panthers (tank that benefits the most from Blitz) it's deflection more often than not. Not to mention that even reaching the throwing range can be tricky.
I've been thinking about the blitz issue a lot lately and this is the main reason why I think it should be disabled when engine is damaged.
AT grenades are not 100% guaranteed a damaged engine on panthers, don't know if they're even 100% on Pz4's either.
It's not like it's super easy to cause engine damage unless you charge the tank with conscripts, as you are sure to take losses if you do something like that.
A soviet player should get some kind of benefit from sacrificing his men to get engine damaged, blitz disabled is one way to do it.
I'd prefer to disable the blitz ability completely for one reason:
The damage an AT grenade does is pretty much nothing, you can repair that damage in no time plus the fact that it's not a guaranteed engine damage either, especially on panthers who benefit the most from this ability.