In my experiences with Phals in s/h some of the roots do die off & some don,t.
If the plants are in high humidity then new root growth will take about a month or so.If the roots reach almost down to the res than the size of the pot is correct.Potting roots actually in the res normally means they,ll rot quickly.Roots that actually grow into the res will be fine.Some people water plants every day when they,re establishing themselves & i have done this with a Paph with no rot problems , just make sure there is a good air flow on the plant to dry off any that splashes on the plant.
You can unpot the plant after a few months & remove the dead ones.Regular flushing of the pot will wash a lot of the debris out of the pot & i have had no problems with plants that haven,t been repotted.
Once the roots reach the res then the plant seems to go into a root growing overdrive & my Phals have filled their containers with roots in less than 6 months.
You may lose a leaf or two if you,ve trimmed the roots .