We just need a compose key, stop trying to do better.
Meanwhile, in a Spanish QWERTY, reinforced by its requirement to support Catalan and Galician, ç and ñ have their own keys which can be uppercased as normal, and the acute `, grave ´, circumflex ^, and diaeresis ¨ all have their own deadkeys which can be used with any vowel character, uppercase or not.
It's not totally arbitrary. They are used a lot, while ï and ê are less used.