Assuming the frame also is PT wood, that ought to last quite a while, but ultimately they too will deteriorate, notably in the posts at the water line or where the tops of the joists are attached to the posts. I had to disassemble part of the frame on my old dock a couple of times late into its life to replace a post, and it wasn't fun. Ultimately I replaced the dock with an aluminum frame crankup after 20 years of lugging the heavy frames out of the water each fall. The dock was getting old, but I'm far older.
If ultimately you want to replace the deck, for either a wood or aluminum frame, sections can be made of PT wood, cedar, or synthetic decking. PT sections are medium weight (but you already know the weight of your sections if they are PT), and PT is fairly cheap. Synthetic decking (eg. Trex) is more expensive and much heavier, unless you opt for an even more expensive but lighter weight product. In the end, I built mine out of cedar, which has a natural rot resistance and is fairly light. I weighed one of mine, six feet wide and a few inches short of four feet out; it was 57 lb, and quite easily moved. Synthetic decking would have been around twice the weight (ugh). Cedar comes in a grade called STK (Select Tight Knots); likely you'd use 5/4 x 6 planks (1" thick, 5-1/2" wide). It can be stained/painted for appearance, but as that wears regular reapplication would be needed. Cost of cedar is quite reasonable.
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