I mean that all dates (I should say: in "occidental" countries, countries which used a Julian calendar at the time - I'm not talking about Asia...) up to 4/14 October 1582 were recorded at the time using the Julian system (the only one used in Occident at the time AFAIK). And the convention has remained, since then, to keep the julian as official dates. Go to the Wikipedia entries for any notable or royal of that era, you will see that the dates are Julian.
I believe translating those dates (before 15 October 1582) to proleptic gregorian on Wikitree profile pages - on the data fields - would be extremely confusing because anyone looking at the profile who is familiar with the person profiled would say "Ah! But this is wrong! He was born/died/married 10 days before that!".
I have no problem with adding a proleptic Gregorian conversion in the biography, or anything that you suggest above.
All that I'm saying is that, on a pre-1583 profile without more information, the overwhelming odds are that the dates use the JULIAN convention.
Just to make it clear, I would fully support having several calendars available, and indeed it is the only solution to make things clear for the long period (1582-1923) where Julian and Gregorian calendars co-existed.
Clear as mud, right?