The reason LLY, BIIB, PFE, etc love mAbs is that biologics outlast their patent lives for nearly forever, whereas solid dosage products have one or more generic competitors set to launch upon the minute of patent expiry. As you note, in all other respects, the anti-beta amyloid mAbs carry several anchors dragging behind them. And when patients start dying of brain bleeds, it is going to be bapineuzumab all over again.
Yes, those companies should buy Anavex out for $10b today while they can. But, they probably won't because they just aren't that smart (as institutions - individuals there are brilliant of course.)
Rett will likely gain approval pretty quickly. And AD will follow right behind. It would not surprise me to see significant off-label PD use because reps will be detailing neurologists who not only see AD and Rett patients but also PD patients - and those reps will hand out the biomarker PD and AD gene up and down regulation data. Revenue should come quickly in all markets where 2-73 is marketed.
Then, once all the risk is wrung out of Anavex and the price per share is extremely high, and only then will LLY etc. want to buy Anavex. Big pharma companies are very risk averse.