Rifles can freely push them, Tommies can freely poke them from range.
So let's examine this.
Let's say 1v1 Ost vs USF, USF has 2 paths Ost plays vanilla.
early game: 3 rifles vs 1 mg42 3 grens
You stick the Grens in green cover positions and wait for them to spot incoming riflemen. Your job as Ostheer is to spot the incoming rifles and position the mg42 properly to suppress them and force critical retreats, aided by higher unit count early before 1st officer and the 42 sight pios.
USF has no support weapons early so the regular build is 3 riflemen. He has to take great engagements where the 1v1 or 2v1 with your grenadiers starts at the lowest range possible in order to deny the Ost player turning his MG to fire at him. This is the only thing USF can do early, concentrate a force on one area to overwhelm your unit(s) there with their superior infantry, all while playing around your mg42 which they also need to spot with their standard sight range to best avoid it. Game sense plays a huge role for both players, as does positioning.
LT: LT and .50cal hit the field, LT possibly getting a zook once 222 hits the field. .50cal is in support of Riflemen. Ostheer gets T2 and 222 hits the field. Next units to hit the field are Stuart and pak40. In this pre-stuart phase, you have a good 1-1.5 min window with the 222 (depending on mp trades and map control of course) to bleed vet 0 rifles and support your grenadiers, taking a sizeable amount of map control thanks to your earlier LV powerspike. As Ostheer you also get techless weapons upgrades at this point, either an LMG42 that bleeds riflemen excellent early game if you play positioning properly or 5man Grens which are zombies that don't bleed and can play much more aggressively than LMG Grens. USF then hits the Stuart powerspike, gets LV supremacy back, and Ostheer is confined to one half of the map mostly (you can still harass other side) so that the pak40 is always near the action to not let the much more mobile stuart freely bleed your 30mp reinforce infantry and kill your 222.
Cpt: Same scenario as before, except Cpt gets early BAR, AAHT hits the field faster, reducing your 222's window, followed by M1 ATG to further confine your 222 and Pack howitzer to force Ostheer to play less static to avoid bleed.
In my opinion if LMG Grens and unupgraded Grens could solo fight off riflemen like 5man Grens can, the Ost vs USF matchup would be a joke. Grens need to be weaker 1v1 than riflemen (at least in certain scenarios, later LMG Grens are better at range than any rifle M1919 + BAR or double BAR) in order for USF to be able to fight.
Where one need one unit, ostheer need two. Lets say cons vs Volks match up, even if cons would have to retreat after engaging volks at close range, volks would most likely have to be retreated aswell. Its a small but win. Both USF\UKF can engage unsupported grens, with minimal casulties.
As I was saying before, on paper and as a raw stats unit, grens are fine and good. But in reality they are put in such environment vs USF\UKF that playing with them is just painfull, considering you are in constant full disadvantage if you are not bringing some specific toys.
Well I don't see why you'd be playing with just Grenadiers. You have a T0 MG that's the best defensive MG in the game, you have a sniper that hits the field 1min into the game, you have a mortar in T1, you have an early light vehicle at 0 muni investment, accessible healing, excellent tech with no tech choices and everything available, a workhorse medium that hits the field in excellent timing, great doctrinal support. You have a much better roster than your opponents and are encouraged to use combined arms to win. In contrast, UKF lacks a lot of basic tools and has to compensate with strong infantry sections, fast UC and AEC. I don't like their design, but it's not an excuse to buff Grens so they can 1v1 sections. Same with riflemen.