As noted Scourge does have decay/upkeep however part of playing on a high pop server such as US Scourge is that many build locations may be limited.
If you'd like more options to build you can check out EU Scourge which has a good ping for US players as it's on the same network. There's also AU Scourge which may have even more locations available.
You've noted that a few stone bases decay after 7 days, this could be the inactivity purge removing inactive player and bases from the map which will also take care of any inactive player base regardless of materials.
It's not the role of the volunteer admins to tell players how to play or build, admins do check out areas with spammed tc's that claim a large amount of unused land (if you see them let one know.) It's always best to upgrade a base as it furthers gameplay but this isn't always an option (player may only be on for that day or have pc issues etc.)
I do think upping twig decay is sort of a good idea and it does get mentioned often however it would impact new players just starting out and maybe add to current outpost vending machine material issues (which is already under strain from networth farmers.)
In the survival/scourge servers it's popular thing to do (especially around monuments) to have TP bases. Which are usually just a foundation, tc and bag. It's small on upkeep and land cost which allows more players to build around and in my opinion doesn't break the rule of claiming large areas of unused land. It can be a lazy way to travel and it negates the use of maintaining a car, horse or not exploding a mini (it may work better for players who have pc issues and can't use these transports due to low fps.)
As it gets closer to end of wipe, areas may open up if your fast enough to snatch them up when they become available.