What exactly means stockpiling ? Opening additional LIRs to satisfy one
organisation's own need ?
No, to "stockpile" is to store something without using it. "Need" doesn't come into it.
In what is stockpiling "last /8 space" worse than "stokpiling IP space"
in general ? Especially for already-acquired space ?
What is done, is done.
Stockpiling *more* space means there will be even less even more quickly for future LIRs, or fewer future LIRs, and it also increases operating costs, because the IP space available on the market becomes less even more quickly.
You seem to think it's a problem that already-acquired space is "unused", why don't you think it's a problem that even more space stays "unused"?