Well, I don't like the idea of spending $750 on a water heater intall "to see if it fixes the problem". The problem could be clogged nipples at the inlet or outlet of the WH, or a clogged or partially closed shut-off valve on the WH inlet. It is true that a 10 year old WH may fail tomorrow, or it may have 5 good years left on it. I would not spend a lot of money fixing it, but the 2 items I mentioned could be checked or repaired for a reasonable amount, although there will be a minimum service call charge. Other that the 2 items mentioned, it is unlikely that the WH itself is the cause of low pressure.