I don't feel like power play is the best defense. It's got to be the most ignored feature, one which no one asked for to begin with. Are CG's really as susceptible to last-minute stocking? It doesn't really seem like it… CG's in my experience have been big enough Affairs that one extra run does not a huge difference make. And considering that both sides of any conflict can use instant transportation… Does it make a real difference? However, rep given.
I'm sure you just asked for:
Can you justify an argument against instant transfers without A) bringing in an argument against transfers in general such as jump distance, and B) bringing up role play which we are not all into.
I'm pretty sure that meets those requirements, regardless of your particular view of PowerPlay. There are a lot of pretty involved players who play PP and if FD don't mind about further breaking the balance of PP then they should probably just remove that until they can add a mechanic in that they do want to support. It seems odd to alter the balance of a game mechanic for what they admit is a last minute "QoL" addition.
CGs are just as effected - a hauling CG is still time-dependant and competitive, it's not so much about last-minute sniping but if haulage capacity is increased per-player that's going to make a difference in situations where groups are trying to stop people hauling into a CG. There have been blockades of CGs before and this makes opposing the delivery of goods harder because more can be shipped in more quickly. Even if you don't think PowerPlay is an important mechanic, presumably you accept that CGs are relatively popular?