Imo, czerny is far from out of date. Ofc, this is obviously personal, but here is why:
1. In basically all classical pieces you play scales, all kinds of arpeggios, trills, chords, and loads more...
Czerny has got short exercises for all of those things, that will help you develop it.
2. In romantic music you have the same thing, plus a billion other things.
Czerny's got it.
To say that czerny is a waste of time, and that you only need one exercises, is, for me, like saying that the chopin etudes aren't that great after all.
However, this is personal. I practise like 10 hours every day. I have had time to practise a lot of Czerny, and therefore think they are good.
If you practise, say, 1 or 2 hours, 100 Czerny is probably a bit over kill, and you should focus on only a few essential.