What are the implications of no longer requiring a complex aircraft for the commercial/CFI checkride?
First, as Ron pointed out (and as the FAA Notice you quoted says), you still need 10 hours complex (or turbine) training time for a commercial single-engine certificate. The only change is that you no longer need to do the checkride in a complex aircraft.

Your main question seems to be if it will eventually be possible to instruct in a complex aircraft without having any complex experience. That could indeed happen if the FAA ever implements a proposal to allow Technically Advanced Aircraft (TAA) time as an alternative to complex time for commercial single-engine. Then you could fly a Cirrus (or another fixed-gear TAA) instead of something complex and become a CFI with no complex experience. That proposal requires a change to the regulations so it's a bigger deal than just changing the ACS and it hasn't happened yet.

But, it's already the case today that instructors don't need to be endorsed in the aircraft they instruct in: if they aren't acting as PIC then they don't need to be qualified as PIC. (Whether doing that is a good idea or not is another issue.) So instructors without endorsements wouldn't be a new issue at all, although it might become a more important and common one.

However in reality, insurance often determines what an instructor can do, not the FAA. I fly a C182RG, which is a complex aircraft, and the insurance policy specifies the experience requirements for instructors who use it. In a hypothetical future where you can become a CFI with zero complex time, you simply wouldn't meet the insurance requirements to provide instruction in that aircraft. I know insurance isn't mandatory anyway, but for CFIs and other professionals it's often the deciding factor in what they can and can't do.

As for unintended consequences of this change, I have no idea and we can only speculate. For example, complex single-engine aircraft prices may fall as flight schools sell them off.