Difference between promptness and promptitude

Seeing this EL&U question: ‘saying thanks to someone answering your email ASAP who is important for you’, the first word that came to my mind was promptitude which, as the definition states, can convey both punctuality and non-delay-ness(immediate-ness)

So I thought this would be a good answer for said question

thank you for your promptitude

But after googling the subject (to see if this is an american expression as required by the OP), I stumbled upon the word promptness which seems to convey the same meanings of punctuality and non-delay-ness(and to be a lot more popular than my first frenchy choice). So, I thought that one could equally/better propose

thank you for your promptness

Now, I have no problem recognizing promptness as an equally/more valid word for the same usage. But, at the same time, my problem lies precisely there: it is too much an equally valid word for the same usage, because when I went hunting for small nuances between the two words(because someone on EL&U/ELL once said that synonyms usually denote of different connotations) I couldn’t find any.. I am further appalled by the fact that they stem from the same root.

Yes, the question:

Am I wrong in :

1. assuming that promptitude & promptness have no distinct connotations (in this context particularly)?
2. thinking that perfect synonyms are something of a rarity?
3. thinking that same-root perfect synonyms are something of a rarity?
4. a combination of 1., 2., 3. and some other speculation of mine?

Why? (added for the charmingly joking nature of EL&Uers)

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Edit

After receiving some comments and one answer I have to add:

Again, I have no problem recognizing promptness as an equally/more valid word for the same usage.

But it seems to me that we have these two words from the same root that mean the same thing. How come? Is it just because one is becoming obsolete and that’s how words die – another one comes and slowly but steadily takes its place? or is it something different?