- If the city did not have a monastery before discovering Scientific Method, it will never be able to build missionaries. Sorry Canada, no spreading Christianity for you.
Maybe obsoleting a building should also obsolete the requirement to have it? That should be relatively simple to implement.
My initial thought re: the monastery discussion: in RFC:Europe, monasteries get replaced with seminaries as the 'modern-age religion building boosting science' -- could that be an option here? Modern seminaries are often where missionaries get trained, so they'd fill the same role in that sense as well.
Another point: in general, it does feel pretty weird that it's specifically 'Scientific Method' that obsoletes monasteries, when it was famously a medieval monk (Albertus Magnus) who pioneered empiricism and the scientific method, and it was a modern monk (Gregor Mendel in the mid-1800s) who discovered genetics using the scientific method. Not to mention that it was monasteries exchanging letters on their experiments and observations that inspired the circular letters and periodicals (and proceedings and journals and eventually peer-review) of the early Enlightenment scientific academies that (iirc) are enabled by the tech in the game.
It
does make sense for monasteries to obsolete in the modern age, since they are far less relevant today than they were to the Middle Ages, and it
does make sense to set that obsoletion at Scientific Method, though purely for game balance (the same tech adds & removes science bonuses, which avoids the exploit of beelining Scientific Method while never obsoleting monasteries to maximize science for as long as possible). So I get it, but it still rubs me the wrong way -- it's sufficiently ahistorical that there should be a better way to get a similar effect.
One final thought: in the game, 'missionaries' are mostly used for converting your own cities, or occasionally converting foreign cities of friendly powers (to get a shrine boost or positive relations). This is pretty close to historical behavior; there were a few missionaries who did long-distance missions, but they were far rarer and more notable (and more easily represented by Great Prophets, frankly). So the current use of missionaries fits the historical behavior... until we reach the modern era.
Once we hit the modern era, missionaries -- at least those from Christian Europe -- tended to focus far more heavily on overseas work, and far less on local evangelism. Many (most?) famous explorers of the early modern period were in fact missionaries by profession ("David Livingstone, I presume) or acted like missionaries while exploring (e.g. Magellan converting the king and queen of Cebu in the Philippines). In more recent years, current missionaries tend to focus heavily on social work -- growing up in the US, I've seen many churches who celebrated and donated to missionaries specifically for building schools and teaching literacy, or building wells to provide reliable fresh water, etc. (FWIW, there are quite a few NGOs that were founded as missionary organizations that do similar philanthropic work).
I feel like this aspect of modern missionaries is missing from the game at this point. As above, this certainly makes sense -- this element doesn't really fit with the historical behavior of missionaries which is the game's focus, meaning that 'modern missionaries' would effectively be a whole new gameplay element. I also feel like such an overhaul would probably fit well alongside the archaeology & exploration & natural wonders overhaul that's already been discussed before. So it might be best to put it on the back-burner for 1.19, but since the topic was raised I did want to mention it here.