True, you'd adjust for an improved FTP, but if you're doing the same intervals at the same %, you're going to get the same intensity factor, and training stress score. If you just keep repeating that, you're going to stagnate.

Most trianing plans work through a progression where you're building base, then raising FTP, then working on V02 max, then working on anarobic, sprinting efforts. So you're always going to new intervals targeting a new area.

And even while you're in a phase targeting a particular area such as power at FTP, a good plan will mix up the intervals, one to keep it mentally fresh, and 2, as Clausen said to keep your body challenged.

So for example I went through a block recently where the first week was 30minutes at 85%, straight into 8minutes at FTP. Next week was 2x20's at FTP, Next week was 4x8's at FTP, but with 2 30 second bursts in each of the 8 minute intervals.

All those intervals are aimed at raising FTP, either by pushing it up, or pulling it up, but you only do each of them 3 days, and you're on to something else next week.