We are living in a complex world of supply chains and what not. You can't always forsee everything in terms of timeline and what all can happen when you are building up a new productions of devices.
I don't know, why you are complaining so instantly. I am pretty sure that RME would be glad in the 1st place, if all the planned timelines would have become true, simply because its their business and they earn money with it.
In regards to the backward compatibility of ARC USB there were surely reasons not to tell it too early.
I could think of that also this required development and a lot of testing to ensure, that this is working.
And the UFX II is very similar to the UFX+, so maybe this was easier to be kicked out quickly to market, as UFX+ was already out.
Be fair .. ARC USB is there soon and its also a very nice present from RME to all customers, that they
get many years later after purchase of their old product new functionalities without having to upgrade their hardware.
So I think you should relax a little. I am also waiting for the ARC USB and will simply be happy once its there.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13