Imo the only mainline infantry that should have sandbags are those that benifit from them via a cover modifier.
Cons (pre 7 man it's essential for their intended role and poor stats, and after the upgrade for their bonus)
Tommies, again cover bonus. Although I'd toy with the idea of moving the trench to sappers and making it more attractive somehow
Ostroppen, yet again a cover bonus. Similar to cons their job is to be holding ground not taking it.
All engineers on the flip should have access and any other mainline access should be doctrinally gated. Cover is important and should require sacrifice to generate, not on the units you are using to take ground through brute force as you want them anyways. Make unit composition more important and viable.
Agree with all the above - but then grens should have it without doc as they will never be stock 5 models squad. One thing to be remembered - even if they get sandbags it is (in my opinion) not going to infuence the game much. It will probobly make the game less micro intensive for ost (no need to run around with pios that much) and let players surprise opponents with grens' positioning (not as predictable as map cover). It's just a detail that will make game better and reward skill. It may mean that some players will come back to building grens instead of skipping them. To make it even better you could remove bunker building ability from grens. It could be reserved for pios.