I liked it because it’s the correct answer, but I wish it weren’t true.
What would possibly work would be a rune upgrade path.
4 common of the same can be combined to make four rare. Four rare could be combined to make one epic. Four epic to legendary, 4 legendary to mythic.
For runes with different secondary still be able to combine them, otherwise the possibility of getting anything more than epic is basically zero… But if you don’t use all the same secondary, you have no guarantee what secondary effect you get.
Maybe even go one more level and combine 4 mythic to get a mythic for the white spell (either random or you can pick of there is more than one white version). It wouldn’t be cheap, but it would be something to work toward. Especially since some of those white spells are almost impossible to get runes for.
This idea is not original with me.