Some mainline it makes sense to be able to build cover. Cons for example are meant to be utility units that hold the line. Cover makes sense there. Same with Tommies whom have worse stats when not in cover and horrible moving stats and no sappers until tech. They are static infantry so it makes the utmost sense for them to be always able to fight from cover.
Ost on the other hand is as combined arms as it gets. You should always have pios on hand and it's always worth having pios on hand. There is no real reason for grens to build their own cover tbh and if it's on grens then what happens if you plan to go ass grens? No sandbags for your mgs? No. It doesn't make sense.
I agree only to a point I'm afraid. What you wrote about sappers coming later for UKF makes perfect sense - tommies need cover. But they retain this funcionality throught the game. It is so much more convenient to use the unit that needs cover to build it itself. In real game situations it could be a gamechanger as sappers will be repairing vehicles, sweeping for mines, using ATs on tanks and won't need to help mainline infantry. This is the problem with grens and generally ost. They will need to stop repairing a tank to build sandbags for example. It is not fair and much more micro intensive. Think when you have 3 tommies. They will build their cover very quickly while their sappers will place mines. Ostheer will have to wait for three sandbags and the mine 4 times longer.
It should also be remembered that at some point in the game tommies will become 5 men and will still build cover (grens won't).
Grens are even more horrible on the move than tommies - trust me on that
I can't understand why you want ass gren to build cover - they don't normally need it. They are not a stationary unit. I don't see a problem here. Still you can always give this possibility to them.\\
I don't like tbh when everybody keeps repeating that ost is about combined arms as an excuse for making their units/abilities worse. Yes, it is about combined arms... ...but so are all other factions. This is actually the biggest logical mistake I keep reading here over and over again. At the beginning of the game ost has access to mgs and pios. After it builds the first structure you get grens, sniper and a mortar. This is when UKF ar SOV can field similar units. USF must wait a bit longer for their mgs. But it is just the beginning of a match. Very quickly from the match beginning all factions have all combined arms tools that are basically of similar quality. Yet, ostheer is probably the only faction that has to rely more on them as their core infantry is inferior. Their support weapons aren't any better.