This is VERY annoying. Having too many of the shards and cores can be seen as a luxury. But since you can really only own a small number in the inventory that is shared for three characters, there are really too few available.
Lets say that the new season have some new exotics and some new armor type you will need to thrive in whatever new game mode, or whatever random armorstat gets nerfed/buffed for pvp (this we know can happen anytime). The inventory we have is too small to be able to freely create masterworked builds for all characters. The possibility to grind for shards is also very limited at the beginning of seasons.
The postmaster ones get deleted everytime you happen to run a few strikes/gambit/crucible matches ( all showering you with umbrals and blue gear, overfilling postmaster without any warning).
The very limited amount of the most difficult to obtain currency is just a way to create an artificial scarcity, to make sure that players are always grinding. But this is just cheap and poor design, decided by some behavioural analyst to keep people hooked to the game. What they dont realize, is that we who play destiny a lot does it no matter what the grind target is. I (barely) tolerate the grind, but most enjoy destiny when I can just chill around in pvp with my friends, and dont have any stupid pinnacles or currencies to chase. At the other end are those who play casually, joining in for the latest content and leaving as soon the game goes from a new experience to a repetitive grind. Hence their casino style rng drop and limited ability to stack up on resources are just an annoyance that does not bring the game anything positive. But some guy hired by corporate to "boost player engagement" says that it is good so that's what's in the game.
I would love to have no limits for shards, since then I can earn them while I want to, and use them when I want to. But requiring us to upgrade and delete crap gear at a net loss just to be able to reset the rank and have a chance for a good, high stat exotic is beyond stupid design.
Destiny is seriously becoming more and more difficult to tolerate, the more they try to boost player engagement by putting artificial hoops for us to jump like good little dogs.
What I don't get is why the postmaster will push out exotics to make room for blue trash that should be able to be auto-deleted. Why doesn't Destiny do what Borderlands 3 does with its lost loot machine and prioritize loot? It's a really much better system on a game with much better attention to detail in my opinion.