I hate seeing spells in my spellbook that I know I'll never use because they've been eclipsed by higher level spells; it seems such a waste. I think all of the spells in your book should be useful no matter what level you are. Scaling is a great way to do this. An archmage might even find Grease to be the best spell to use for his turn if it covers a large enough area to make all of the enemies fall. (not actually sure if that's a good example since I didn't look up if it does scale, oh well, imagine your own example if this one doesn't work ) If lower level spells become more powerful for high level mages, then this might need to be balanced by reducing the number of spell slots per level, but I'm okay with that.
I also really like the idea of metamagicking spells, thereby making them a higher level.