Kot, three words: Close quarters combat.
Step 1.
Procure RPG-7.
Step 2.
Screw off the protective cap on the end.
Step 3.
Stab the enemy with your new spear.
Step 4.
? ? ?
Step 5.
Profit!
On the more serious note - it's getting modern now, bayonet charges and hugging the enemy is getting a bit stale. And even then, I'll rather slam a man with my bazooka club than M203... something, also I don't exactly see how you're planning to use a grenade launcher in anti-tank role...
atleast two forms of armor piercing rounds, one using magnesium based steel and the other manganese based
I mean, explain to me how do you expect a solid slug you proposed as a functional anti-tank projectile at... what, 75 m/s? Granted, you mentioned use of HEAT, but considering that grenades usually lack any kind of stabilization (as in, those tumble a lot during flight), it's not going to be accurate enough to use shaped charges with good effect consistently.
Grenade launchers dont have balckblast, and all the issues that has. Even things with limited blackblast (say, the rhino recoilless rifle of cheese) only have that by launching something out the back that you do not want to be in the way of, and in rockets its a big ole burst of fire.
Let's just replace the RPG-7 initial explosive booster with
PIAT style lauching system if the existence of backblast is that much issue for you. Or
this. Also, theres indirect fire, which grenade launchers can do and rockets cant really, along with versatility. Rockets are slightly more limited than grenades, mostly because they are a fair deal larger and heavier than any form of grenade, so its less effective to have things like flares, smoke, and similar rockets to hand out to infantry squads
As for indirect fire - well, we had our mortar back during the Forenian wars that was basically rocket launcher. And even then, with enough skill you could theoretically use the rocket launcher as one in the same way you want to use a grenade launcher. And even then, rockets lead to missiles and missiles are the most indirect-fire weapon that exists. As for weight and size - let me put
this here again. Okay, granted, it's still bigger than a grenade but it's practically a tie with a grenade launcher (compare loaded weight of M79 that is nearly 3kgs versus a little over 2kgs of Komar).
Yes, we want rocket launchers someday. But we also want grenade launchers, and probably just normal grenades someday.
Maybe, it's not like that I don't enjoy blasting people with GMGs, especially since they're the closest feeling to firing a bolter you can get, but rockets and missiles will be more useful. We need to first make rockets, which will (maybe not perfectly, but always) fill the niche of grenade launchers and partially grenades, and then we will need to convert them to stuff like MLRSes, guided missiles, rocket pods that we could mount on planes or helicopters or whatnot. Grenade launchers are cool, but we have limited amount of design actions so we have to squeeze the most out of them to be better than those... other guys (who are we fighting again?)