I honestly think this is one of the most reasonable ideas. Twig is the most laggy, and most giant bases are made of twig, because upgrading buildings is expensive. This should both the size of bases (or at least the number of giant bases) because not everyone will want to grind materials for wall upgrades. And it should limit the number of laggy and ugly twig only structures.
The only structures that shoudl have uncnditiona decay on server should be non TC structures. If you can't be bothered to make a TC you clearly don't need that extra building.
Edit: I really beleive that turning on decay for all building types, will only make more players build from twig only, as twig is the cheapest to upkeep. This seems to have a potential to cause even more lag, and make the server look like absolute crap with all the twig around. The quastion you need to ask is: do players care more about the size or the look of their bases? Because if the answer is "size" (as I believe it is) then more people will opt to sacrifice the look in order to be able to afford the upkeep of their base. And therefore more people will build with twig only.
