Pd's are not the issue at hand here. The problem is that you can spam RAS bois that give you economy but if your enemy decides to push you already have BP in place (Ras coms) that are hard to kill and can easily build tons of PD's.
The problem with the later stages of the game is that the amount of mass you have in units compared to your income is growing. Basically usually the longer the game goes on the longer it would take for your economy to re built the army you have. This indirectly makes reclaim insanely important and thus promotes very static gameplay.
I think that in order to achieve healthier gameplay in later stages of the game we would need to encourage players to trade more during the game. The easiest way to do that would be to reduce the amount of reclaim you get from higher tech units.
We should look over some games and see how the value of Total mass in units/mass income changes through the game when we reach different stages and then adjust the reclaim in order to bring values of different stages closer together.
I would need to go and see some replays but from the top of my head the difference rn is huge. (This is for 10x10 land maps, different maps have different values and it also depends if you make land, air, navy).Early t1 stage is ~2k/~25, Late t1 stage ~4k/~35, t2 stage ~8k/45, late t2/t3 rush ~14k/~60 , t3 stage ~20k/80, late t3 stage ~30k/100. As you can see from this very inaccurate approximation from the top of my head the value of the units on the field grows much quicker.
To address one of the arguments against changing our beloved 81% reclaim value. I really disagree that changing that number for different tiers will confuse ppl so that they won't know how much reclaim they will leave. First of all most people have no idea how much units cost so they don't know how much reclaim a Rhino will leave anyways. Changing the % of reclaim left by t2, t3 units won't really confuse ppl since it's quite intuitable, higher tech units leave more mass, it doesn't matter (in sense of understanding) if a brick leaves 800 or 1k mass, it leaves more then your t2 tank, that's all you really need to know. If we are talking about judging how much mass will a failed attack donate you do it with experience and on the fly judging so people will just adapt with time to those changes.
Another counterargument is that with such changes all the wreck props (reclaimable units) on the maps will be changed with such balance change. I am really not to sure what's the best way to address this issue but I don't think it should stop us from balancing the game. Possible solutions are: Just let it happen, most maps will be absolutely fine with slight reclaim decrease. If author wants to adjust his maps, he will do it, as for GPG maps and Classic ones whose authors are inactive we could look over them and see if any would require changes and adjust the wrecks there as needed.
The other idea is to separate the values for civilian owned units or w/e but I am not sure if this is even possible and it's extremely hacky and then unintuitional for future mappers, so I don't think this is the way to go.
Over all I think there is a need for some changes to the values of reclaim in order to make the gameplay especially during later stages of the game healthier, more action packed and rewarding. If we look at other RTS games, especially SC2 we can see similar pattern. Pro matches there used to be either cheeses or games where something happens in the early game, then both players macro'ed, maxed out and clashed once. Right now games can be a 30 minutes of nearly constant fighting, often in different parts of the map at once.
As for the exact number I don't have a proposition yet, I am not even sure if we should balance it for t1, t2, t3 ,t4 or for each tech stage for Land, Air, Navy separately. It needs to follow a logical progression -> higher tech units yield more reclaim (in total since they cost more) but they yield lower % of their cost. With such changes I really hope that we could also buff micro compared to macro since players would be more rewarded for constantly trying to use their units and to find weakness in opponents defense (as it is currently during t1 stage on most 5x5 and 10x10 maps).