Don't need one, nor really want one, but kinda glad someone makes one...

https://www.bearcreekarsenal.com/17-hmr ... upper.html
AR's are cool though with the versatility. Even the short-action AR-15 (vs AR-10) can support a huge variety of cartridges.
I think betwen 5.56/223, 300 Blackout, and 50 Beowulf, the only other one I really have much use for is a 22 LR.
Having said that, since one of the main reasons to have a lower-power rimfire upper would be 'cheaper practice', it might be that for the first 150 yards the trajectory of the 17 HMR would better replicate what the 'real' 5.56/223 does.