Aside from the obvious concession that this will negatively impact all FO's, IF this goes through, the major problem that I see is that there is no incentive for the company to adequately plan ahead for the number of trips that they will need for training.

I'd rather see this not pass at all, but if it must...here's an idea for a small modification:

The company can withhold up to 75% of the trips that they wish, but any withheld trip that does not operate a training rotation must go out as a GS/WS/YS/Reserve coverage with premium pay attached. If this was the case, there would be appropriate incentive for the company to not withhold more trips than they expect to need...