Pilot DAR
Richard Collins did NOT advocate removing the turn and slip, what he did advocate was replacing the traditional instrument with a second AI AND installing a 2 inch turn and bank somewhere else in the panel. Without a turn and slip the aircraft would NOT be IMC legal (IFR in IMC), but might be legal under VFR (see posts above).
You are therefore correct, you need at least a means of keeping the ball in the pot.
With glass sytems the ball is often combined with the backup AI which works just fine.
Richard Collins was however correct. Relying on one AI (particularly if vac powered) is daft - however good our instrument flying skills, it is no fun completing the mission with just a turn and slip.
This makes a lot of sense and provides a lot more useful information.
I dont think it does make sense.
The information you get from an AI doesnt change. The purpose of having a second is to take over from the first, when the first fails. The only extra useful information is the ability to check one against the other BUT you still need to be sure you know which one to believe so there is an additional inherent danger in having two of which you need be aware.