Yes, but that would take more space and take more time to write, and “this replaces a letter” is how a lot of diacritics have appeared. For example, the ñ replaced a “nn” in spanish. This would also remove some ambiguity in pronunciation on words like rathole and foothill.

Also, letter like that are written with dead keys, usually, so it would be more like one new character.

Again, not worth the effort of changing, but it would be an improvement.