There isn't likely to be text/graphic elements less than 4 lines high on HD, hence that issue on 405 / 525 / 635 gone (it wasn't as big an issue on later antialiased caption generators as earlier ones on 625, 525 was worse).
You ARE less able to resolve detail on fast movement, so the the idea of interlace to save 50% bandwidth by reduction of resolution only about 25 Hz was a good compromise.
720p isn't enough resolution for larger screens.
A lot of Cinema content is 24 fps, only recently is 48 fps being used.
It's significant that 480p and 720p are used most for sport in USA and we don't have the equivalent 576p.
It's shame that WS 16:9 was launched before HD and that there MORE HD formats (resolutions & frame rates) than there were Analogue. HD should have been 1152 active lines and 48 fps progressive only. Instead we have five so called HD resolutions up to 1080, progressive / interlaced, 1:1 pixel and anamorphic, Frame rates of: 24p, 25p, 25/50i, 30/60i, 48p, 50p, 60p and interpolated 100p and 120p display, as well as all the PC resolutions and frame rates (43i, 60p, 75p, 5:3, 4:3, 16:10, 16:9, visible lines: 600, 800, 1024, 1200 and higher.) PCs had better than TV HD by 2000, but we have gone backwards on screen resolution on most ordinary PCs/Laptops DOWN to 1080p, which isn't enough for a full page A4 (1200 lines is minimum and we had that in 1990s!). Also 16:9 is a rubbish shape for a laptop screen compared to 4:3.