But I worry about the life of the plastics that are in the thru hulls as I have no idea how long they will last and how they will fail. I have both speed and depth instruments.

I just had an unrelated plastic failure that surprised me... the hinge on my Raritan marine head. I was just cleaning the loo with a clorox wipe when the hinge failed. No doubt part of the reason was due to cracks at the points where screws held the hinge onto the seat.