So true... but, then again, it could lose its value with more runtime. It's absolutely perfect how it it. (Yes, there are minor plot holes, but they don't distract from the overall meaning and impact, they are somewhere in the background) I'm sure Dazai himself would want more time to spend with Oda. But, his life is short, just like the 4 episodes we get. I feel like that's also part of that experience
In the manga adaption of Dark Era, it was shown that Dazai was delivered to Mori's clinic after a suicide attempt. That's when Mori made a deal with him to be the witness for the previous Mafia boss' death in exchange for a pleasant suicide poison. Mori didn't fulfill his part of the deal, though. I read somewhere that Dazai isn't an orphan.