It's really hard to have "realistic" time progression because in reality civilizations had their ups and downs and would not make smooth progress all the time. Just think of how early the Chinese had gunpowder and an administration on a level Europe could not match until quite late. Diseases and other catastrophes which impacted development. Most importantly, and I think that could find its way as a feature into civ games to make them more dynamic actually, wars which have society transform towards a more militaristic orientation at the neglect of arts and culture and other innovation. In Civ 6, your research is never set back or impacted by crisis which shift resources away since the yield from districts works even without specialists (who only give rather small, unimprovable yields), so your development is basically first like ancient Egypt, then like Rome, then medieval China, and from then on the path of post-Columbian Europe into the present.
To make the game more realistic in the long term, science would have to be much less of a steady stream and tied more to excess resources being made available which can only be done at special times (i.e. it would be riskier during war), and technologies should not be handled in such a strict propietary manner anymore as it was introduced with civ 5 and instead allow more spillover through trade routes, diplomatic trade, and general proximity to other civs and local diversity. The Eurekas are a nice step in this direction as well, I think.
Until then, we'll try to stretch the values to European history I guess, but for that civ 6 would first need to get the techs into a decent order (like not having the Ruhr Valley right after renaissance and flight no longer before chemistry)