Why would the power supply have to be replaced?
Because at 6-7 years, the power supply....like the hard drive...is usually already heading downhill. A replacement power supply is usually about $30 or so. If you are going to go to the trouble to open up the Time Capsule.....a difficult task....it would make sense to replace the power supply at the same time. Your call, though.
I only have a 1TB that I know is reliable
This would be a SATA drive, correct? Apple used several different drives in the same 802.11ac Time Capsule, so you won't really know whether you have a WD or Seagate drive.....or another drive....in the Time Capsule until you open it up. Space is really tight.
If your 1 TB drive is not a "green" drive, it will likely place more demands on the old power supply than the original drive.