There is a reason the Umarex Colt's forward assist is fake. A forward assist on a Rimfire would just be a second means of firing the chambered round.
The Umarex Colt is probably the most accurate rendition of the AR, but it's a garbage weapon. All looks, no functionality. Also, none of the other AR look-alikes, like the 15-22, (Bushmaster, Umarex, etc.) can utilize actual AR hardware and furniture like the 15-22. S&W did an amazing job at recreating the AR experience with their design. They also removed the items they believed to be useless, like the dust cover and forward assist. Could they have left it in the design? Sure... but all it would be is something that could get broken that they would have to fix, or added weight, or a big one... added cost. Granted we all spend hundreds (if not thousands in some cases) of dollars outfitting our 15-22's.
The .22lr round was never meant to be used for long distance precision or tactical target elimination. Even with new barrels, an alloy upper, etc. I don't see some amazing increase in accuracy for the weapon.
The demand for a more accurately designed .22lr AR-15 in APPEARANCE only is because some shooters enjoy making their weapon indistinguishable from an actual AR. Why? I'm not 100% sure... I would assume these are the same guys I see at the range in full tactical gear that are about 100lbs overweight. Meaning... they try to look the part, but it just doesn't work. Much like a .22lr that replicates an AR 100%.
Like anything, it's a matter of "I want it because I want it". Personal preference. It serves no purpose other than to satisfy the need of the shooter to give an appearance of firing a real weapon and not just a recreational firearm.
If you want to spend that kind of money for an AR that fires .22lr ammo, then buy a real AR-15 and a .22lr conversion kit. Or a .22lr upper and put it on a real AR lower.
For that matter, you could do what Umarex did yourself... go buy an Airsoft AR-15 and shave off the fake forward assist and dust cover... then epoxy it to the M&P. I'm sure some could waste the time to fashion one that just superglues on, like I've seen the dust cover that was created by members here.
The bottom line is... in reality, I am not knocking the ingenuity of the members here. On the contrary, I've seen some great ideas and solutions come from the posts on this forum. I mean, almost every picture I've seen in threads here, everyone has some useless equipment or unnecessary equipment on their 15-22. We all (including myself) attempt to make our weapon emulate the AR platform. If we didn't, we wouldn't have chosen the 15-22. In reality, there's really no need for a forward vertical grip on a weapon with little to no recoil... There's no need for tactical BUIS or a muzzle break that gives the appearance of a suppressor... Or in some cases, optics that are designed for CQB, or distances that surpass the ballistics of the weapon or the projectile. Unless there are a rash of Zombie Squirrels in the trees around your home that just HAVE to be eliminated immediately with a $15,000 night vision optic on a $600 weapon. But it is awesome to see that on the rifle! We're creatures of "upgrades"... just look as some of the vehicles on the road today and you can see that! Everyone want's to make their unique mark on their "stuff". It's just what we do.
We do it because it's fun and entertaining and to be the one to get that "Hey, your 15-22 looks awesome!". It's the same in the AR community on other forums. It will always be that way as long as we are still free to do as we please (in most states).
I say If you want a forward assist and dust cover... do what you can do to make or fashion one for the weapon. I think that's the beauty of this weapon... there are ways to modify it beyond most's capabilities.
On another note, if you truly believe non-functioning aesthetic devices on your weapon help you train for use with your actual AR, then I say go for it. I am a firm believer in you can never have too much training. However, if that is your goal... Then I would definitely recommend just converting your AR platform to fire .22lr ammunition. You won't have the accuracy, firing a .22lr from a .223 barrel, but you would definitely have the realism.
If your goal is accuracy... you've already got about as good as you're going to get with the 15-22 at 50 to 100 yards.
I absolutely love my 15-22... and again, I do outfit my M&P to imitate my AR-15, but I am not going to put a $1200 quad rail or a $3,000 optic on it either. (Well, maybe for a photo here or there).
So... I'm sure if the demand calls for it, someone (like a member did with the dust cover) will create a snap-on version for those who would like double takes at the range. I usually get them more-so with my converted AR-15 than with the 15-22 though.
Oh, and like all great communities, you guys are innovative as hell!!! Some of the designs and modifications I've seen here are just incredible. Also, the knowledge base of users on literally every individual part of this weapon is staggering!!! lol... you guys are great!
Enjoy!