My vote....walls won't be changing....even if Firaxis changes something in BTS.
In fact I like the idea so much about having buildings go obsolete that it needs done more.....you snooze you lose type of thing. It is a dynamic in game play and style that I surely don't want to lose and probably will enhance.
I am not the last word here and the Team likes to hear everybody's input....but I can be quite persuasive
I agree, more buildings should go obsolete, but have a replacement/upgrade option. In this way, your city is building upon what it already has, instead of basically losing buildings with no benefit. For example, I believe the real reason monasteries go obsolete with scientific method is to take away their +10% research, as science moved into non-religious hands. This has nothing to do with the ability to build missionaries, in fact even in our modern times there are still plenty of missionaries out and about.
I think it would be great to have more buildings "upgrade" to replace themselves when they go obsolete. For example, monasteries go obsolete, removing all benefits, but then you can build an upgraded monastery aka 'Seminary' for a small amount of hammers (or gold?). A seminary, for instance, would train missionaries but have no bonus to anything else (maybe culture?). If you didn't have a monastery in the city and you are past Scientific Method, you can still build a seminary, but for a much greater amount of hammers.
Other ideas:
Walls obsolete City Guard obsoletes, upgrade to National Guard Base.
Monument obsoletes, upgrade to Statue Garden or Art Relic
Musket Manufacture obsoletes, upgrade to Rifle factory (maybe go backwards and have a building 'Fletcher' (for archers) obsoletes, upgrade to Musket Manufacture)
TNT Factory obsoletes, upgrade to Explosives Factory or Missile Factory
Stable obsoletes, upgrade to Tank Factory