The storage of the castings makes a big difference in how 'good' the microbes are. Do they all die? I seriously doubt it, but they are certainly not as good as found in castings kept slightly moist during storage. If I was using older castings, I'd simply brew the tea a little longer to let the microbe populations increase.

You can brew worm tea but I have found a better method. It takes months to make worm castings. I have 5 worm bins made from Kitty litter buckets and 2 inside bins (rubbermaid containers). I have a lot of potted plants and hated tossing the spent potting soil (expensive) into the garden. I have 2 compost tumblers and when the exothermic reaction is done I add worms to speed up the process. All bins are raised and have collecting trays. The drippage is the best worm tea. I collect it daily using a turkey baster,bulb syringe, or toomey syringe if you have one. You can collect the tea long before the castings and pour it on your potted plants. I have a 5 foot cilantro that really loved it and have brought back many nutrient deficient plants. Recycling potting soil in worm bins really works too and the worms love them.