For starters, the entire science fiction premise is not to be taken as a significant plot point. This isn't a hard sci-fi story where universal laws are well established and explored within the themes of the plot. Worrying about why they have expiration dates completely misses the allegorical nature of the plot. Isla is no different from a human girl. The giftia recovery process is nothing more than a thinly veiled excuse to have the protagonists introduced to the larger themes of the series before Isla herself becomes the focus. Trying to world build until the very end probably would have turned out horribly. I mean, the Matrix was a fantastic Sci-Fi movie but look what happened when they tried to dig deeper into that!
Even more crucial is that giftias having souls is a stated premise of the show. Isla's robotic behavior at the start is intended to be allegorical. Or maybe it's just a giant, awful pun. Either way it doesn't bear scrutiny and doesn't have to. People become withdrawn and afraid of emotional attachment for many reasons and Isla has several reasons to pick and choose. This is what made the series stand out from other death-tragic-romances for me; the intentional focus on the development of the person who is dying rather than solely exploring the grief of the survivors.
Tsukasa, ironically, nearly becomes a manic-pixie-dreamboy in the process, which is probably why the side characters spend so much time nearly breaking the fourth wall with their invasive and unhelpful advice. Having him called a lolicon, the various pickup-artist attempts he makes and everyone protecting Isla's feelings at the start all make sense within the context of the characters while also allowing Tsukasa to prove his sincerity to the audience.
I won't bother with numbers here, because this is a beautiful series despite its clumsy delivery, shallow side characters and weak ending. I forgive it for its painfully accurate portral of regret, depression, fear of emotional attachment and misguided delusions of a better future. Ironically this is probably a series best enjoyed by people who have honest regrets about wasted time/youth.
PS: Hey, there's a lesbian who didn't get senselessly murdered or rejected!
