First, volks do not currently carry the late game. There is a reason why double obers is extremely common, volks fit their needed role to a T-hold the line for obers to do damage. Second, there are absolutely units out there that have general purpose all range weapons and are not super elite uncommon units. Pretty much anything with semi automatics fits this description. And to be honest, even with the STGs, I'd say volks have more emphasis on long range anyway,just as pgrens have more emphasis on close range but are capable at long.
Your second paragraph ends with a statement that highlights why I think volks current niche is fine. OKW already has a specialist close range(sturmpio) and specialist long(ober). Giving volks mp40s steps on sturms toes. Ironically you can see an similar situation with I/R stgs. They are uncommonly purchased because volks already fit that combat niche, one that ober LMGs do not.
As for scaling, I already addressed this. The best comparison is to assault grenadiers who have sprint which is an excellent compliment to their weapon kit, have superior vet and can upgrade to 6 man in the late game which further boosts their scaling and they STILL dont scale that well. Smoke grenades are great but that doesnt change the fact that SMG units scale poorly. This is also notable with PPSH cons, though their superior recieved accuracy through vet and lower cost to begin with means they are at least better as a meat shield.
STGs are a no more brainless upgrade then BARs, LMG34, DP28, or 7man conscript. And I dont see how the fact 7 man comes late makes it any less brainless. Your argument is that the MP44s are a boring upgrade, how does their timing change that-or rather how does timing make the 7 man upgrade any more "interesting"? (Hint, it doesnt but it's no less necessary for it)
The other element is that if MP40 volks become a thing to have players choose between long range and short range, then K98 volks will need a substantial buff, and not just via vet. They will need either better combat stats out the gate, or reduced cost, or an upgrade that gives it to them (like that 7 man con upgrade) for the exact same reason cons recieved it-poor late game scaling for the basic bolt action mainline.
And even if this way is done, obers will be a better choice than k98 volks at all ranges, and sturms better then MP40 volks at all ranges. It's just a lose lose.