I'm not sure if the OP was about dwindling numbers or the feedback idea. Since the dwindling numbers part felt like he was just throwing out random numbers and percentages to try to make his case I'll just skip that part.
Yes, the numbers were just estimated. It differs from server to server. One server lost 60% of it's player base and another server lost 80% ... there could be an average number calculated but I had to watch tons of server to get a good average.
The feedback idea you had though would be doomed from the start as you described it. Essentially you are punishing older players for playing on the server. The longer they stay, the more they get held back. The system you suggest to solve dwindling numbers would soon be added to the causes of dwindling numbers.
Feedback systems aren't an idea I came up with. It's an concept you'll find in every game, intended and unintended. While positive feedback systems happen naturally, negative feedback systems are added to games to make them more fair. Think about the most MMOs. The starters have a starting area and their PvP flag is off, so high level players can't kill them for fun. In some games high level players can kill low level players but there are penalties for it. Making c4 harder to obtain would already be an negative feedback system, because newbie bases wouldn't be raided that often. The Lockpicks are a negative feedback system as well, because you can't loot a naked guy right away and you don't want to wait, so why bother shooting in the first place (I don't like this feature though, because of the downsides).
I don't want to punish older player for playing on the servers - I rather want to help new ones to be able to get started.
The large, good equipped group of players should focus on taking down other large, good equipped groups of players and not on some newbies. At the moment you can raid a small newbie house and get more resources out of it as you used for the c4, which is a really bad thing imo.
If you want to keep players, there needs to be the opposite, give them incentives to stick around, even after being raided. Reward the players for playing on the server longer, and encourage new players to stick with it. The problem isn't just players getting left behind, it's also that a lot of players give up a little too easily.
That's right, you have to encourage players to stay on the server. The only progress that isn't lost when you die is the stuff you can craft - if they add the feature that research kits destroy the researched items and you need more than one kit, this will already encourage the players. If this change happens, you worked hard for the ability to craft C4 or a M4 or some other rare items - you don't want to research it again on another server.
When house ownership is implemented and doors can't be replaced by non house owners it encourages the players as well. It's no problem to fix up the base but when other players placed doors in it, you just have to start over. I currently have four bases, so I'm not completely lost when someone claims my house. I got raided six times and they didn't get much because of the multiple bases, since I don't keep everything in one place. It's currently the only way I can think of to survive on a two week old server.
I just think a new player should be able to build some stuff before he gets destroyed and this will be achived by making the new player less valuable for the older player.