I really disagree that OKW doesnt have an upperhand over those two in early game/mid. Do rifles fight well against volks early? yeah decently enough. Does that mean okw is just autolosing early game? not at all, because they have kubels, sturms and other supplemental units. No mainline should have sandbags, if you want to buff volks RA by 10% or something to compensate, go for it. But as it stands, it completely nullifies map design and other game factors that have been addressed here.
It seems like we all agree sandbags on mainlines isnt good for the game or how it was meant to be played, but the hang up is happening with people that play one specific faction. To me, thats no excuse to make the game healthier, just give volks a bit better early game firepower if thats that big of a deal(which i really think it isnt, just use map cover like everyone else, yes this goes for cons too).
Yes, Rifles walk over Volksgrenadiers, they don't merely "fight well against them". The M1 Garand, when compared to the (volk) K98 has a (very significantly) better damage profile at any range up to 23, where Volks then eke a sub-0.1 DPS advantage up to 35. This ignores that Rifles also have better RA than Volks, however, which means Rifles ostensibly win at any range, and crush Volks flat close up . Volks 1-on-1 (Or any equivalent engagement) can only perform vs rifles if they force the rifles to close in on them from superior cover, and manage to take out a model or two before they reach them. This is why Volks currently have Sandbags, they do not fight efficiently without them. A "10% RA buff" isnt an equivalent exchange, and doesn't promote the same gameplan as current. As Gachi has mentioned, buffing Volksgrenadiers has the risk of ruining the OKW vs SOV matchup, which is generally pretty balanced.
To keep pace with USF infantry OKW must use Sturms, but especially in 1v1s Sturms cannot be in more places than one, and Sturms aren't something you can afford to build multiple of. They're expensive, take a lot of population, and dont scale to be effective enough in the lategame.
The Kubelwagen is also great, but is quite micro intensive, requires frequent tending to by Sturmpioneers (Who have other shit to do) and doesn't scale terribly well in combat terms (Though a Kubel in the mid/lategame is still valuable thanks to its speed, low population requirement, and ability to both cap and detect units)
UKF, OKW, and SOV can't simply have their mainline sandbags taken away without serious thought being done to adjusting their unit performance. OKW and SOV especially don't have overpowered mainline infantry that need to be made to perform worse. UKF arguably do, but their factional design is abysmal, they still crutch far too hard on Sections, and consequently they seriously need major, major changes. The problem with all of that is its a ton of work, and the Balance Team have limited time, resources, and permitted scope from Lelic.
As mentioned multiple other times, incidentally, OKW can't afford Sturms being their only source of Sandbags. There's a reason you don't see multiple Sturms in most games. They would need some new supplemental unit to assist in building cover if they even wanted to be on the same level as other factions in this regard.
Removing sandbags from mainlines is a great idea, they ought to be something engineers build. There is a lot of work to do before you can just up and make that change though.